Bob has some 10 lb weights and some 3 lb weights. together, all his weights add up to 50 lbs. if x represents the number of 3 lb weights and y represents the number of 10 lb weights, which equation can be used to find the number of each type of weight bob has?
a)3x – 10y = 50
b)3x = 50 – 10y
c)50 + 10y = 3y
d)50 + 3y = 10y