SOLUTION: Aquilla buys 50 stamps, some for 10c and some for 20c. If she spend 6,40 altogether, how many 10c stamps does she buy?

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Question 960482: Aquilla buys 50 stamps, some for 10c and some for 20c. If she spend 6,40 altogether, how many 10c stamps does she buy?
Answer by CubeyThePenguin(3113) About Me  (Show Source):
You can put this solution on YOUR website!
x = number of 10c stamps
y = number of 20c stamps
x + y = 50
10x + 20y = 640 ---> x + 2y = 64
x + y = 50
-x - 2y = -64
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-y = -14
y = 14
She bought x = 36 10c stamps.