SOLUTION: Two sisters, Mary and Jane have a total age of 98. When Mary was Jane's age, Jane was half as old as Mary is now. What are their ages?"

Let M = Mary's age now
Let J = Jane's age now
Let x be the number of years ago it was when Mary was Jane's age

That says 

M + J = 98

That says "x years ago Mary was Jane's age now"

M - x = J

That says "x years ago Jane was M

J - x = M

So we have this system of three equations in three unknowns:



If you don't know how to solve that system, post again asking how.

Answer: J = 42, M = 56, x = 14.

Checking:  

That checks because 56 + 42 = 98

x = 14 years ago Mary was 56 - 14 or 42, Jane's age now.  That checks.

x = 14 years ago, Jane was 42 - 14 or 28, and indeed 28 is half as old as
56, Mary's present age.

So that is correct.

Edwin