Question 173305
Let {{{n}}} be the number of 3 cent stamps, then the number of 8 cent stamps has to be {{{3n-1}}}, and then the number of 15 cent stamps is {{{(3n-1)-(6)=3n-7}}}


The value of the 3 cent stamps, in cents, is {{{3n}}}


The value of the 8 cent stamps, in cents, is {{{8(3n-1)=24n-8}}}


The value of the 15 cent stamps, in cents, is {{{15(3n-7)=45n-105}}}


The sum of these values is $2.47, or expressed in cents, 247 cents, so you can write:


{{{3n+24n-8+45n-105=247}}}


Simplified:


{{{72n-113=247}}}


{{{72n=360}}}


{{{n=5}}}.


So there are five 3-cent stamps.  I'll leave it to you to figure out how many 8 and 15 cent stamps based on knowing there are five 3-cent stamps.  You should also check the work to make sure your answers fulfill the requirements of the problem statement.