Algebra, Age Word Problems

Question

Mark's sister is 8 years less than twice his age. If Mark is 39, than how old is his sister?

Answer 1

Let Mark age be X and his Sister age be Y

Mark's sister is 8 years less than twice his age

Y = 2*X - 8

If Mark is 39, than how old is his sister?

So X = 39

Y = 2*39 - 8 = 78-8 = 70