In a chemical reaction, substance A combines with substance B to form substance Y. At the start of the reaction, the quantity of A present is a grams, and the quantity of B present is b grams. Assume ab. At time t seconds after the start of the reaction, the quantity of Y present is y grams. For certain types of reactions, the rate of the reaction, in grams/sec, is given by
Rate=k(a−y)(b−y)
where k is a positive constant.

For what values of y is the rate nonnegative?
I thought the answer was [a,y]U[y,0], but this is incorrect.
Any suggestions? Thanks!