The cost of making a dress is partily constant and partily varies with amount of time it takes to make it. lf it takes 3 hours to make it cost $40. lf it takes 5 hours to make it takes $46. Find its cost if it takes 1 1/2.
c = a + bt
They tell you that
3a+b = 40
5a+b = 46
so, a = 3 and b=31
now finish it off