Question 212916
Let {{{f}}}= cases of fax paper
{{{c}}}= cases of copier paper
given:
(1) {{{f + c = 250}}}
(2) {{{17f + 11c = 3650}}}
Multiply both sides of (1) by {{{11}}}
(1) {{{11f + 11c = 2750}}}
Subtract (1) from (2)
(2) {{{17f + 11c = 3650}}}
(1) {{{-11f - 11c = -2750}}}
{{{6f = 900}}}
{{{f = 150}}}
And, since  {{{f + c = 250}}}
{{{c = 250 - f}}}
{{{c = 250 - 150}}}
{{{c = 100}}}
There are 100 cases of copier paper
check answer:
(2) {{{17f + 11c = 3650}}}
{{{17*150 + 11*100 = 3650}}}
{{{2550 + 1100 = 3650}}}
{{{3650 = 3650}}}
OK