Question 943086
First, define your variables clearly. What are x and y? To minimize confusion, let b and s be Bob's and her sister's ages now.


b = s+15 (Bob is 15 years older than his sister)
b+10 = 2(s+10) (Bob, in 10 years, is twice as hold as his sister, in 10 years)


Replace b with s+15 in the second equation and solve for s.