Question 161980
Let t be the three cents stamps, e be the 8 cents ones and f be the fifteens
Given {{{e = 3t-1}}}
{{{f = e -6}}} so {{{e = f+6}}}
which says {{{3t-1 = f + 6}}}
so {{{3t - 7 = f}}}

{{{247 = 3t + 8e + 15f}}}
Now using the values for f and e in terms of t above, substitute and solve
{{{247 = 3t + 8(3t-1) + 15(3t -7)}}}
{{{247 = 3t + 24t -8 + 45t - 105}}}
{{{360 = 72t }}}
{{{5 = t}}}
Now use this value for t to find f and e