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Let *[tex \Large x] represent the number of 20 cent stamps.  Then the number of 15 cent stamps must be *[tex \Large x - 5].  Represent the total value of the stamps as 345 cents so that we can work with integer coefficients.


The value of the 20 cent stamps is then *[tex \Large 20x] and the value of the 15 cent stamps is then *[tex \Large 15(x - 5)], and the sum of these two values is:


*[tex \LARGE \ \ \ \ \ \ \ \ \ \ 20x + 15(x - 5) = 345]


Solve for *[tex \Large x]


John
*[tex \LARGE e^{i\pi} + 1 = 0]
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