The gap is always 33 years. Her mum is three times her age when that gap is twice the girl’s age, so when she’s 16 and a half and her mum is 49 and a half.
A girl of twelve asks her mum for a bicycle. Her mum, who is 45, says, “You can have one when I’m only three times as old as you.” How long does the girl have to wait?
Give me a hintThe hint
The gap between their ages never changes.
Reveal the answerHide the answer
Four and a half years
Source: Henry Dudeney, Amusements in Mathematics, 1917, no. 45, Mother and Daughter