Homework Help: math

Posted by Sandra on Wednesday, October 17, 2012 at 12:02am.

If paint is sold for $ 15 a can and $20 a can and they sell 320 dollars worth of paint, how many cans of paint do they sell? I know it takes 7 cans of paint at 20 equaling 140 and 12 cans at 15 equaling 180 but I', stumped at how to write the equation

math - Anonymous, Wednesday, October 17, 2012 at 12:35am
Let x = 15$ paint
math - Reiny, Wednesday, October 17, 2012 at 9:17am
Number of $15 cans --- x
number of $20 cans --- y

15x + 20y = 320
divide by 5

3x + 4y = 64

we are looking at the equation of a straight line, where the solutions can only be integers, (can't buy a partial can of paint)
x-intercept = 64/3 , x < 21
y-intercept = 64/4 , y < 16

3x = 64-4y
x = (64-4y)/3
64-4y must be a multiple of 3
with y as an integer, 64-4y can only be
60 56 52 48 44 40 36 32 28 24 20 16 12 8 4

of those the only multiples of 3 are:
60 48 36 24 and 12

so, assuming they buy at least one of each can, possible solutions for (x,y) are
(20,1)
(16,4)
(12,7) ---- your answer
(8,10)
(4,13)

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