GoalSubtract three- and four-digit numbers and check each answer by adding.
New words
difference · the answer to a subtractionopposite operation · an operation that undoes another one, as adding undoes subtractingcheck · to test an answer in a different way
Explanation
Subtraction finds the difference. We can check a subtraction by adding. Add the answer to the number that was taken away. You should get back the number you started with. For 846 − 325 = 521, check 521 + 325 = 846. We can also subtract by counting up from the smaller number to the bigger one. To find 703 − 258, count up from 258: 2 to reach 260, 40 to reach 300, 400 to reach 700 and 3 to reach 703. The jumps add up to 2 + 40 + 400 + 3 = 445, so 703 − 258 = 445. Check: 258 + 445 = 703. If the check does not give back the first number, look for a mistake.
Worked examples
Place
Subtract
Result
Hundreds
800 − 300
500
Tens
40 − 20
20
Ones
6 − 5
1
846 − 325 by place value. Subtract the hundreds, tens and ones: 800 − 300 = 500, 40 − 20 = 20 and 6 − 5 = 1. So 846 − 325 = 521. Check: 521 + 325 = 846.
912 − 387 by counting up. From 387, jump 13 to 400, jump 500 to 900 and jump 12 to 912. The jumps are 13 + 500 + 12 = 525. So 912 − 387 = 525. Check: 387 + 525 = 912.
A farm's hens laid 4,860 eggs in two months, and 2,350 of them in the first month. How many eggs in the second month? 4,860 − 2,350 = 2,510. Check: 2,510 + 2,350 = 4,860.
Class activity
Subtract and check. Write three subtraction problems with three-digit numbers, such as 803 − 456. Solve each one, then write the addition that checks it. Swap with a partner and check each other's work. Which answers needed a second try?
