3
Taking away tens and ones
Discover Mathematics P2: Term 1, Lessons 31–40
GoalSubtract two-digit numbers by taking away tens and then ones, and estimate first.
New words
subtract · to take one number away from anotherdifference · the answer when we subtractestimate · a close answer found quickly
Explanation
To subtract two-digit numbers, take away the tens and then take away the ones. For 37 − 25: tens 30 − 20 = 10, ones 7 − 5 = 2, and 10 + 2 = 12. Before you solve, estimate with the tens. For 59 − 16, think 5 tens take away 1 ten is about 4 tens, so the answer is about 40. The exact answer is 43, which is close to 40.
Worked examples
48 − 23: tens 40 − 20 = 20, ones 8 − 3 = 5. 48 − 23 = 25.
Farida had 75 beads and used 41. Tens: 70 − 40 = 30. Ones: 5 − 1 = 4. She has 34 beads left.
Estimate 43 + 42 with the tens: 40 + 40 = 80. The exact answer is 85, which is close to 80.
Class activity
Guess and check. Pick two numbers from a calendar or bus ticket, like 63 and 31. First estimate their difference with the tens. Then work it out exactly and see how close you were.
Practice
1
What is 86 − 32?
54
2
What is 67 − 25?
42
3
What is 90 − 50?
40
4
Mariam had 78 stickers and gave away 54. How many are left?
24 (70 − 50 = 20 and 8 − 4 = 4)
5
Estimate 63 − 31 with the tens.
About 30 (60 − 30). The exact answer is 32.