Subtraction in the Real World

Activity symbol Activity: Making Change

Suppose you have $25.90 in cash (two $10 bills, five $1 bills, and nine dimes) and you owe a friend $16.87. How will you break the $25.90 in order to pay off your debt?

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Discussion

Start with the $16.87. You can give your friend a $10 bill. What will you still owe her?

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Answer

After giving your friend $10, you will still owe her equation left hand side dollar times 16.87 minus dollar times 10.00 equals right hand side dollar times 6.87. You still have five $1 bills, so you fork those over, too! Now you only owe her equation left hand side dollar times 6.87 minus dollar times 5.00 equals right hand side dollar times 1.87.

Since your friend won’t budge, you need to make change for your remaining $10 bill. Let’s say you break it into a $5 bill and five $1 bills. Then you hand over $1 to your friend.

At this point, your debt is now equation left hand side dollar times 1.87 minus dollar times 1.00 equals right hand side dollar times 0.87. After giving your friend eight dimes, the amount you still owe is equation left hand side dollar times 0.87 minus dollar times 0.80 equals right hand side dollar times 0.07.

Taking your last dime and trading it for a nickel and five pennies, you hand over the last 7 cents you owe. Now you’ve erased your debt!

Subtracting One Number from Another