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The Case of the Lunchroom Losses

Caleb Craft is a young detective with a passion for math and a thirst for mystery. Can you match wits with him?

“Oh no, not again!”

“What’s the problem, Mrs. Clabber?” Caleb asked. 

“The register is 55 cents short!” Mrs. Clabber exclaimed, shaking her head. 

“Can I take a look?” Caleb said. He noticed that the shiny register had the words AUTOCHANGE 2020 marked on the side. 

“This register is new. It dispenses change automatically,” said Mrs. Clabber. “I always have the right amount of change in the drawer at the start of lunch. But by the end, exactly 55 cents are missing!”

“Don’t worry, Mrs. Clabber,” assured Caleb. “I’m on the case!”

The next day, Caleb dashed to lunch. He stood next to the fruit shelf, peering at the register.

Caleb watched the students pay for their lunches. Their change clinked automatically into the register tray. The prices of each item and the change given back added up correctly.

It was five minutes before the bell was about to ring, and everything seemed fine.

Suddenly, 11 girls carrying field hockey sticks entered.

“That’s the field hockey team,” Mrs. Clabber chuckled. “They always come to lunch together. They get the same thing every day!”

Sure enough, each of the 11 girls picked out a single cup of $1.75 cheese fries. The team captain, Marcie, handed Mrs. Clabber two $1 bills to pay for her fries. But the register didn’t give the change Caleb expected. 

Caleb watched closely. The same thing happened when the other 10 girls paid. Two $1 bills went into the register, but it didn’t give back any quarters. 

Caleb questioned Mrs. Clabber about it.

“The machine always runs out of quarters just before they arrive,” Mrs. Clabber said. “But it’s programmed to use smaller-valued coins instead.”

“Hmm,” Caleb pondered. “I’ll be right back.”

Caleb found Marcie at the condiment station. “Hey, Marcie! Could I take a look at the change you just received?”

“Sure!” Marcie fished in her pocket and pulled out two dimes and two nickels.

“Did you have any coins before lunch?” Caleb asked.

Marcie shook her head. “Nope! Our team is doing a fund-raiser. All of our change goes into the jar in Coach’s office.”

“Aha,” exclaimed Caleb.

Mrs. Clabber had the register open when Caleb came back. “Another 55 cents gone,” she sighed.

“And I know why,” Caleb grinned.

“Why?” asked Mrs. Clabber. 

Caleb nodded at the AUTOCHANGE 2020. 

“Turns out your new register has been making donations to the field hockey fund-raiser! You’ll have to get it reprogrammed, and I can tell you why.”

Continue the Learning Journey
Think Like a Detective

Follow these steps to crack the case of the missing lunchroom money.

How did Caleb crack the case? Use your math wits to figure out how the lunchroom money went missing. Click on the slideshow above to help you solve the mystery!

Bills and Coins

Watch a math helper video about how to use bills and coins.

After watching the video, pretend you manage a school cafeteria. Create a lunch menu of foods that you would serve, and write the price for each. Then ask family members to act as students and buy the food items with bills and coins. (If you can’t use real money, make your own with paper and pencils using the video as a guide!) Then practice counting the money and giving your customers the right change!

Ice Cream Shopping

Use bills and coins to pay with exact change at an ice cream shop.

You have three minutes until the ice cream shop closes! Play the game above to help customers pay for different ice creams with exact change. How many customers can you serve?

This article was written by Jessica McKenna-Ratjen for DynaMath magazine.

Image Credits: Illustrations by David SanAngelo