No,” Mrs. Ramirez said, shaking her head emphatically. “I definitely did not write down the combination. Anywhere.”

Corey nodded. “That’s what I told them.”

“We just have to cover all the bases,” Ben said apologetically. “It’s good to know the thief couldn’t have found the combination written down.”

Hannah, Corey, and Ben were in Mrs. Ramirez’s classroom. She had been grading quizzes when they found her. But she was happy to pause a little while to answer their questions. She wanted the missing money returned just as much as her students did. And grading quizzes wasn’t her favorite activity.

“And so you had the combination memorized,” Hannah said.

“Yes,” Mrs. Ramirez answered. “And it was easy to remember.”

“Did you tell anyone the combination?” Ben asked.

“No,” Mrs. Ramirez said. “But I did tell my students the formula for the combination.”

“Yes, Corey mentioned that,” Ben said. “What was the formula?”

Mrs. Ramirez got up and walked over to the dry-erase board. “Well,” she began, “every year with my homeroom class, I use this formula as a learning exercise.”

She picked up a marker and then wrote “First number = even.”

“The first number in the padlock combination is an even number,” she said.

“Okay,” Ben said, writing in a small notebook he carried with him at all times so he could keep track of clues.

Mrs. Ramirez wrote “Second number = (First number/two) squared” on the board.

“The second number is the first number divided by two and then squared—multiplied by itself,” she explained.

“Got it,” Ben said, writing.

Finally, the teacher wrote “Third number = First number + Second number.”

“The third number is the sum of the first two numbers,” she finished.

“Oh yeah,” Corey said. “I remember this now. I didn’t know the answer the first time, and I don’t know the answer now.”

Corey sometimes thought his brain just wasn’t made the right way to get math. But then he’d look at some NBA player’s stats, and he’d understand them perfectly. So maybe it wasn’t just his brain. Maybe it had something to do with what he was interested in.

“There are several possible correct answers,” Ben said.

“Yes,” Mrs. Ramirez said. “That’s right.”

Hannah carefully looked at what Mrs. Ramirez had written on the board. “It seems to me that the formula would be easy to figure out if you knew one of the numbers,” Hannah noted.

Mrs. Ramirez agreed. “If you were just trying to guess the answer, you’d have to go through quite a few combinations. But if you knew one of the three numbers, you could easily figure out the other two.”

Ben thought for a moment. Then he asked, “What’s the last number?”

Mrs. Ramirez hesitated. “I’ve always kept that information a secret. But I suppose it doesn’t matter now.”

“Now that the thief knows your combination, I guess you’ll have to change it, anyway,” Corey pointed out.

She nodded. “Yes, that’s right. So I might as well tell you three. The last number is 35.”

“So the combination was 10, 25, 35,” Ben said quickly.

Corey’s mouth dropped open. “How did you figure that out so quickly? Did you secretly use a calculator?”

“No,” Ben said. “It’s just algebra. Think of the numbers as X, Y, and Z. Then plug in 35 for Z, and solve for X and Y.”

“I think I’d rather just think of your brain as freakishly big,” Corey said.

Hannah asked Mrs. Ramirez if Ben was right. She nodded. “Absolutely. That’s the correct answer, and the combination that opens the padlock.”

“So Ben was able to figure out the combination once he knew the last number,” Hannah said. “But how would the thief have known any of the numbers?”

“Do you say the numbers to yourself as you turn the dial on the padlock?” Corey asked. “Because maybe the thief could read lips.”

Mrs. Ramirez smiled. “Well, I’m not completely sure, but I don’t think I move my lips as I turn the dial.”

Ben got an idea. “Did you spin the dial after you locked the padlock?”

“You mean when I put the box away on Monday morning?” Mrs. Ramirez asked. Ben nodded. She frowned, trying to remember.

“That’s quite a few days ago now,” she said, thinking. “I can’t really be sure whether I spun the dial or not. Do you remember, Corey?”

Corey thought back to last Monday morning. He vaguely remembered Mrs. Ramirez putting the metal box in her desk drawer after everyone had put their envelopes of money in it, but he didn’t remember whether or not she spun the dial on the padlock.

“I really don’t remember,” Corey said. “She could have spun the dial on the padlock after it was below the top of her desk, so we couldn’t see.”

“I don’t think I would do that,” Mrs. Ramirez said. “Once I picked up the metal box, I would probably hold it in one hand while I opened my desk drawer with my other hand. I wouldn’t have a hand free to spin the dial on the combination lock.”

Ben and Hannah listened to all this and thought about it. “My guess is,” Ben said slowly, “that you didn’t spin the dial on the combination lock after you closed it.”

“So the dial was still set on the combination’s final number, 35,” Hannah added.

“Right,” agreed Ben. “All the thief had to do was look at the dial to see the final number. Then he or she would just have to use the formula to figure out the first two numbers in the combination.”

“Okay,” Corey said. “But that means the thief had to know the formula. And Mrs. Ramirez only tells the formula to the kids in her homeroom. Right?”

Mrs. Ramirez nodded.

“Except the fingerprints already showed us that the thief is probably someone from outside our homeroom,” Corey reminded them.

Ben and Hannah looked stumped for a minute. Then Hannah remembered something. “Mrs. Ramirez, didn’t you say that you use this formula with your homeroom every year?”

“Yes,” she agreed. “I’ve been using it for several years now.”

“I see where you’re going with this, Hannah,” Ben said. “Maybe the thief was in Mrs. Ramirez’s homeroom in a previous year.”

“Right!” Hannah said. “Like last year, so you might still remember the formula. The thief might be an eighth grader.”

“Maybe we should fingerprint all the eighth graders!” Corey said.

Ben held up his hands. “We really just need the kids from Mrs. Ramirez’s homeroom last year, but I think we should run this theory by Miss Hodges before we try to get permission to fingerprint any more students.”

“That sounds like an excellent idea, Ben,” Mrs. Ramirez said.

“Well, what are we waiting for?” Hannah said. “Let’s go!”

She hurried out of the classroom. Ben and Corey followed her. At the door, Corey turned back to
Mrs. Ramirez. “Thanks, Mrs. Ramirez!” he said.

“You’re welcome!” she said, turning back to grading the stack of quizzes with a little sigh.