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Let x be the number of 3-sent stamps.
Then the number of 4-cent stamps is (x-3), according to the condition.
x pieces of 3-cent stamps have a total face value of 3x cents.
(x-3) pieces of 4-cent stamps have a total face value of 4(x-3) cents.
Therefore, you can write the equation for a total face value of all stamps as
3x + 4(x-3) = 65.
Simplify and solve for x.
3x + 4x - 12 = 65
7x = 65 + 12 = 77
x = 77/7 = 11.
ANSWER. 11 3-cent stamps and (11-3) = 8 4-cent stamps.
CHECK. 3*11 + 4*8 = 65 cents. ! Precisely correct !
Solved.
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There are other methods to solve this problem.
I showed you the simplest Algebra method here.