2
Spot the mistake
Discover Mathematics P2: Term 2, Lessons 81–90
GoalCheck other people's answers and find and fix mistakes.
New words
check · to look again to see if an answer is rightmistake · something that is not rightcorrect · to fix a mistake
Explanation
Everyone makes mistakes, and good mathematicians check. Here are three ways to check. First, do the problem again, adding the ones first. Second, add the numbers in the other order. Third, make an estimate and see if the answer is close. For 123 + 59, a student wrote 172. Check: 3 + 9 = 12, carry 1, 2 + 5 + 1 = 8, hundreds 1. The answer is 182, so 172 is wrong. The student forgot to carry 1. Mark a wrong answer with an X, fix it, and mark a right answer with a star.
| Problem | Answer given | Right? |
|---|---|---|
| 123 + 59 | 172 | ✗ 182 |
| 98 + 7 | 105 | ✓ |
| 56 + 25 | 71 | ✗ 81 |
Worked examples
123 + 59 = 172? The ones 3 + 9 = 12 need a carry. The right answer is 182, so 172 has a mistake.
98 + 7 = 105? Ones 8 + 7 = 15, carry 1. Tens 9 + 1 = 10, so 105. The answer is right.
Round 35 to the nearest ten. A student wrote 30. But the ones digit is 5, so we round up to 40.
Class activity
Be the teacher. Write five addition problems with answers on cards, and make two of the answers wrong on purpose. Swap cards with a partner. Your partner marks each answer with a star or an X and fixes the wrong ones. Check each other's work.
Practice
1
A student wrote 56 + 25 = 71. Is it right? If not, fix it.
No. 6 + 5 = 11, carry 1. 5 + 2 + 1 = 8. The right answer is 81.
2
A student estimated 52 + 39 as 50 + 40 = 90. Is this a good estimate?
Yes. The exact sum is 91, which is very close to 90.
3
A student estimated 87 − 21 as 80 − 20 = 60. What is the mistake?
87 rounds to 90, not 80. The estimate is 90 − 20 = 70.
4
Check 74 + 18 = 92 by adding in the other order.
18 + 74: 8 + 4 = 12, 1 + 7 + 1 = 9. The sum is 92, so the answer is right.
5
Youssef says 147 + 153 = 290. Is he right?
No. 7 + 3 = 10, 4 + 5 + 1 = 10, 1 + 1 + 1 = 3. The sum is 300.