If Sara's age is exactly twice Bill's age, what is Sara's age?
(1) Four years ago, Sara's age was exactly 3 times Bill's age.
(2) Eight years from now, Sara's age will be exactly 1.5 times Bill's age.
We're given the equation S = 2B
1) S - 4 = 3(B - 4), we now have two equations and two unknowns, that's enough. Sufficient
2) S + 8 = 1.5(B + 8), again two equations and two unknowns, sufficient.