Question

An executive bought 300 stamps for $73.80. The purchase included 15 cent stamps, 20 cent stamps, and 40 cent stamps. The number of 20 cent stamps is four times the number of 15 cent stamps. How many 40 cent stamps were purchased?

I need to do this in a chart form and have already set it up but I am lost on what to do next.
number of 15 cent stamps --- x
number of 20 cent stamps --- 4x
number of 40 cent stamps ---- 300 - 5x

15x + 20(4x) + 40(300-5x) = 7380

solve for x
plug that into 300-5x

Chart form ???
What grade level is this ?

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