How old are they now

Question

Mark is 11 years older than his sister. In 8 years he will be twice as old as she will be then. How old are they now?

Answer 1

m = s + 11

m + 8 = 2(s + 8)

m + 8 = 2s + 16

m = 2s + 8

 

Since m = s + 11 and 2s + 8, they equal each other.

 

s + 11 = 2s + 8

s = 3

 

m = 3 + 11

m = 14