A49 — the tens digit stays the same and you add 3 to the ones
B76 — only the tens digit increases by 3
C79 — both digits increase by 3
D166 — you add all the digits together
Adding 30 means adding 3 tens. The 4 in 46 is in the tens place, so 4 + 3 = 7 tens. The 6 ones have nothing added to them — 30 has zero ones — so the ones digit stays 6. Answer: 76. Option A shows the misconception of keeping the tens digit and adding 3 to the ones instead. Option C shows the misconception of adding 3 to both digits.
Question 2 Multiple Choice
A student says: '54 + 20 = 74, so 62 + 20 must equal 82.' Is this student's reasoning correct?
ANo — the rule only works when the ones digit is 4
BNo — adding 20 changes both digits, so 62 + 20 = 84
CYes — when you add 20, only the tens digit increases by 2, regardless of the ones digit
DYes — but only because 62 ends in 2
The student's reasoning is exactly right. When you add a multiple of ten, only the tens digit changes — the ones digit is always untouched. This works for any two-digit number: 62 + 20 = 82 because 6 + 2 = 8 tens and the ones digit stays 2. The rule does not depend on what the ones digit is.
Question 3 True / False
When you add 30 to 47, the ones digit of the answer is 7.
TTrue
FFalse
Answer: True
True. 30 is 3 tens and 0 ones. Adding it to 47 affects only the tens place: 4 + 3 = 7 tens. The ones digit (7) is unchanged because you are adding zero ones. 47 + 30 = 77.
Question 4 True / False
To solve 35 + 20, you need to check what happens to both the tens digit and the ones digit before writing the answer.
TTrue
FFalse
Answer: False
False. When adding a multiple of ten, the ones digit never changes — there is nothing to check. Multiples of ten have 0 ones, so they contribute nothing to the ones place. 35 + 20 = 55: the tens digit goes from 3 to 5, and the ones digit stays 5 automatically.
Question 5 Short Answer
Why does the ones digit never change when you add a multiple of ten (like 20, 30, or 40) to a two-digit number?
Think about your answer, then reveal below.
Model answer: Multiples of ten (20, 30, 40…) contain zero ones — their ones digit is always 0. When you add 0 ones to the ones digit of any number, nothing changes. All the addition happens in the tens place.
The key is to think in place value, not raw digits. The 2 in 20 is 2 tens, not a plain 2. It can only be added to the tens place. Since multiples of ten have 0 ones, the ones digit is always left untouched.