Bob has some 10lb weights and some 3lb weights. Together, all his weights add up to 50 lb. If x represents the number of 3lb weights and y represents the number of 10lb weights, which equation can be used to find the number of each type of weight Bob has?
(Please explain answer)

A. 3x-10y=50
B.3x=50-10y
C.50+10y=3y
D.50+3y=10y