After checking, Ranran found that the score was what was missing from the Secret. "Even the page numbers match!" Photon looked at it and said with certainty.

"This book is a loose leaf!" William noticed the rings.

"That makes it easier to plug it in again, ha ha." Ranran took the pieces of paper, turned the book to the corresponding pages, and arranged them carefully. Then something magical happened: there were some gilds on the cover of the book, and they shot into the air a beautiful gold light, very similar to the gold light on the outer walls of the concert hall.

"Perhaps the book is grateful that we've found what was missing?" William asked, smiling.

They found the next node effortlessly. The question that awaited them was,

A piano has 88 keys and a harp has 47 strings. In order to reward new and old customers, and promote and deepen the environmental protection process, a musical instrument store launched the "old for new" activity. Here are the rules:

30% off the piano and the harp for $3,500 and $21,000, respectively. One piano can be purchased for every 25 piano keys collected. You can choose parts packages or finished products. One harp can be purchased for every 17 harp strings collected. You can choose parts packages or finished products. In order to ensure sufficient profit, the parts packages will not be sold during the event. If you choose the parts packages, the parts inside can only be replaced once more. In addition, you must exchange them for finished products.

Doolittle now has $200,000, 300 piano keys and 240 harp strings. It is known that he intends to spend 70% of this money on harps, and the rest -- including money not enough to buy a harp -- on pianos. Can you tell me the maximum number of pianos and harps he can purchase and exchange?

"Why is it math again?" Cried Photon in despair.

"Wait, isn't this a cap-for-bottle kind of thing?" William's eyes lit up and he began to read the subject.

"70% of $200,000 is $140,000, which is enough to purchase six harps, with $14,000 left. Add the other 30%, and the total comes to $74,000. That would be enough to purchase 21 pianos, with $500 left." He said, counting.

"So that's all you can buy with $200,000." Natalie followed.

"That's right. You can't buy anymore with $500, not even parts packages." Ranran continued William's line of thought, "Now it's time to exchange. I think Doolittle could trade those piano keys for 12 piano parts packages."

"Why? Won't finished products work?" Photon asked.

"No, no, no, they asked the most. The keys in the packages can still be exchanged, but it's not true in terms of those in finished products. Do you even want to break them?" Ranran replied.

"There will be a total of 1,056 keys in the 12 parts packages. We'll get 42 pianos, leaving us with six keys." She continued, "Well, the purchase and the exchange of pianos are over. He will end up with 63 pianos."

"Now for harps." Said William, "They will make Doolittle get 14 harp parts packages, with two strings left. With the other parts, he now has 660 strings. With these he is able to get 38 harps, leaving 14 strings. He'll end up with 44 harps."

Ranran wrote "63" and "44" on the wall, and the node opened successfully, allowing them to pass.

At the next node, however, they ran into an unexpected problem.

If each stroke of the harp string makes the brush complete a line segment, can this pattern be completed in one try? If so, at least how many times do you have to pluck?

"Wow, that's a nice picture. It's a beautiful five-pointed star!" Ranran's face was brimming with a happy smile.

"Everybody move out the way and let me see." William pushed straight past the others to look at the picture.

He said, "1,2. 1,2,3,4,5,6... Oh, there's no singularity! It can be completed in one try! Let me see. Do I need to stroke at least 11 times? ... Yes, that's it!"

As before, the green light flickered a few times, faded, and the wall opened slowly. This was already the penultimate node.

"This node is so close!" Almost as soon as the wall was fully open, they saw the last green light.

"But this needs us to 'operate correctly'!" Ranran said with bewilderment.

"Let me see what we are supposed to do!" Photon rushed over and looked around.

Then a music box and a harp descended slowly above. Ahead, an area on the wall became a bewildering complex installation. It approached them slowly and stopped firmly in front of them only a few paces or so. It was a curved track, like a black mamba that had just released its bent body, ready to feast on its prey. At the very top of the track was a ball, painted red. A screen descended. It was divided into several divisions, each of which corresponded to one of the strings of the harp.

Then several lights on the track flashed on, and the screen flickered several times. Then a rubber fist reached out from behind and hit the ball, causing it to roll fast.

A section on the screen lit up and William, who was smart and agile, immediately knew he needed to pluck the corresponding string. The harp made a beautiful sound, and it poured slowly into the air like a flowing spring. And with it, somehow, came a unique fragrance. It was like an extract of rose, rosewood and violet mixed together. The moment he plucked the string, the first light flashed. Unbelievably, the ball was accelerated as it passed near it.

"How did that happen?" Ranran asked in surprise and delight.

"It was an iron ball." William said, "And Natalie just saw that there was a magnetic field near the light, which accelerated it."

The ball flew back and forth in the track, and soon came to the second light. This time it was Ranran who plucked the string, and the ball accelerated again. It danced with greater gaiety over the complex stage, sometimes soaring into the sky -- rolling to the top of the circular track -- and sometimes rolling bending its waist -- reaching flat places.

Natalie plucked the string, then Photon. It was William's turn again, and soon Ranran's turn... They took turns to speed up the ball. After about a dozen times, the track could hide its thrilling end no more. After enjoying the sharp turn for the last time, the ball entered the straight ahead and was climbing slowly. The last magnetic field was slowly activated, and that was very close to the end. At that speed, the ball flew away from the finish line without losing much energy. It seemed that the party could hear it laughing loudly in the air; it was actually the rushing wind.

The ball began to descend and hit a red button. They heard a sound "tick", and prompt tone rose, "Ding Dong!"

"Yeah! We did it!" Shouted the party. William and Ranran hugged each other tightly, beaming.

To everyone's surprise, the walls fell down. Ahead of them lay the vast array of instruments. The piano stood in the front, facing them, its lid covered with deep black lacquer, reflecting the light of solemn and noble. Behind it were the cello, viola and violin, each of them standing. It was like three tall pines standing on the edge of a precipice. The harp stood behind them, and on top of it stood a naughty music-box. At the end came the tall trombone, saxophone, clarinet, and a few other instruments.

"Wow! Here they are! Ha ha," Ranran's white face had been occupied by smile. Her bright eyes narrowed, and her red lips were raised, showing her white teeth.

William stretched, "I finally found it. Mr Wilson can finally breathe a sigh of relief! It was really not easy."

But there was something else behind it, and it was dark. They almost missed it.

"It's the missing shadow!" Said Photon, darting straight at it. As it was a blind alley ahead, the shadow could not get out any more, so it had to take off its black cloak and reveal its clothes.

"Ah, this is... Isn't that how a musician looks?" Ranran saw that the man was wearing a neat suit and his shoes were polished to a shine. She cried out in surprise.

The man knelt down, sighed regretfully, then slowly raised his head for a moment, and said, in a tone of some regret,

"I didn't have to do this..."

"What?" William asked him.

"My name is Ben Fred, and I have long heard of this famous concert hall." He looked up and said, "And there are concerts by some of the best musicians, so I often come. As time went by, I became interested in learning music unconsciously, so I would come here from time to time to visit some musicians, hoping to learn from them. But I've been here four or five times without finding one! In a blaze of anger I decided to teach the director a lesson, so I dressed up as a black shadow, stole the musical instruments, and designed such a maze. At first it worked, but since then there have been fewer and fewer concerts. It has often been closed all day in recent weeks. Seeing that the helpless director issued a letter for help to the outside world, I began to somewhat regret my own wayward behaviors. But I still didn't believe anyone could find the instruments. And so my revenge has been successful, until you now stand in front of me, making me remember the story in disgrace..." Then he bowed his head again.

"Don't be sad, Ben. We fully understand your interest in studying music." Said William, patting him on the shoulder.

"Maybe musicians didn't see you because they were too busy composing... But you still can't do that! Look what damage you've done to the concert hall! It's almost closed in here!" Ranran said indignantly.

"I really regret it! I really hope you can take me to apologize to the director, and give me a chance to study music!" Sobbed Ben.

"No problem, don't be sad. Come with us." Photon gently touched his head with the propeller. Natalie came over and looked at him with a smile.