For this case we define the following variable:
We now write the equation that models the problem:
[tex]\frac {3} {4} x + 12 = 39[/tex]
From here, we clear the value of x.
We have then:
[tex]\frac {3} {4} x = 39-12\\\frac {3} {4} x = 27\\x = (\frac {4} {3}) (27)\\x = (4) (9)\\x = 36[/tex]
Answer:
The number of stamps he has is:
[tex]x = 36[/tex]