The leap-day promise

It is the year 2096. Tom, who is 25, loves setting number puzzles at family dinners, and he loves winning them even more.

On 29 February 2096 he makes a promise: 'Give me any age puzzle. If I can't solve it in ten minutes, I'll set no more puzzles until the anniversary of today.'

His younger sister Ada has one ready. 'Once, you were three times my age. Today I am exactly the age you were then. When I am three times my present age, our ages will add up to 100.'

Tom fails. Now he is stuck with his promise until the next 29 February.