brain teaser

[STELLA]

hilda is twice as old as eustace used to be when hilda was as old as eustace is now.if eustace is now twenty one,how old is hilda?

[Stav]

Im guessing 15? Using 21 as the age seems a bit of a pain in the arse, as the calculations I did come out with some fractions, not good in helping decide an age. I got an exact answer of 15 and three quarters years old. Any good?

[mikewall01]

Hilda is 28

[Dsg]

If x is however many years ago that Hilda was the same age as Eustace ie 21 then we have two equations: H-x=21 and H=2(21-x) substituting the first into the second gives H=2(21-H+21) then H=42-2H+42 then 3H=84 and so H=28.