Question 156349
x=number of 3 cents stamps
y=number of 8 cents stamps
z=number of 15 cents stamps

The number of 8 cent stamps is one less than triple the number of 3 cent stamps. 
then {{{y=3x-1}}}
The number of 15 cent stamps is six less than the number of 8 cent stamps. 
then {{{z=y-6}}}

The total value of all the stamps is $2.47.

{{{0.03x+0.08y+0.15z=2.47}}}

we have to find y so we solve for x and z
{{{y=3x-1}}} then {{{x=(y+1)/3}}}
{{{z=y-6}}}
then

{{{0.03(y+1)/3+0.08y+0.15(y-6)=2.47}}}
{{{0.01y+0.01+0.08y+0.15y-0.9=2.47}}}
{{{0.24y-0.89=2.47}}}
{{{0.24y=3.36}}}
{{{y=14}}}

Answer: the number of 8 cent stamps is 14