When I was six, my sister was half my age. Now I am 70. How old is my sister?

How to think about it

1. When the speaker was 6, the sister was half that age: 3 years old.
2. So the sister is 3 years younger than the speaker. That difference stays the same forever.
3. Now the speaker is 70. Subtract the fixed gap.
4. 70 − 3 = 67, so the sister is 67.

Why “67” is the answer

Half of six is three, which makes the sister three years younger, and an age gap never changes as people grow up. At 70 the speaker’s sister is therefore 67. The riddle tempts you to answer 35 by halving again, but “half my age” was only true for that one year.

Common wrong answers

• 35. Half of 70 is 35, but the sister was half the speaker’s age only when he was six, not for life.
• 64. That subtracts six instead of the three-year difference.