Mark is 4 years older than his sister. What are their ages if after 3 years Mark will be twice her age.

Let mark's age be M and his sister's age is
S

Given that mark is 4 years older than his
sister:

Then M = S + 4
............(1)

Now after 3
years:

Mark's age = M + 3

His sister's
age = S + 3

But Mark's age will be twice his sister's
age:

==> M+3 = 2 (
S+3)

==> M + 3 = 2S +
6

==> M = 2S + 3

But from (1) we
know that M = S + 4

==> S + 4 = 2S +
3

==> S = 1

Then the sister's
age = 1 year old

Then Mark is 1+ 4 = 5 years old
Now

After 3 years:

Mark will be 5+3 = 8
years old

His sister's age will be 1 + 3 = 4  years
old