Question 1109457
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<pre>
Let x be the number of the 13-cent stamps.


Then the number of the 24-cent stamps is 2x,  and

     the number of the 39-cent stamps is (100-2x-x) = (100-3x).


The "money equation" is

    13x + 24*(2x) + 39*(100-3x) = 3340    cents.


13x + 48x + 3900 - 117x = 3340,

-56x = 3340 - 3900 = -560   ====>  x = {{{(-560)/(-56)}}} = 10.


<U>Answer</U>.  10  13-cent stamps;  20  24-cent stamps  and  100-10-20 = 70 39-cent stamps.


<U>Check</U>.    10*13 + 20*24 + 70*39 = 3340  cents.   ! Correct !
</pre>


<U>The lesson to learn from this solution</U>.

<pre>
    This problem is reduced to a single equation in one unknown.
</pre>

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