Question 180263
Let {{{s}}} = bushels of soybeans to be used
Let {{{w}}} = bushels of wheat to be used
given:
(1) {{{8.5s + 4.5w = 5.5*1000}}}
(2) {{{s + w = 1000}}}
Multiply both sides of (2) by {{{4.5}}} 
and subtract from (1)
(1) {{{8.5s + 4.5w = 5500}}}
(2) {{{-4.5s - 4.5w = -4500}}}
{{{4s  = 1000}}}
{{{s = 250}}}
And, since
(2) {{{s + w = 1000}}}
{{{250 + w = 1000}}}
{{{w = 750}}}
250 bushels of soybeans and 750 bushels of wheat were used
check:
(1) {{{8.5s + 4.5w = 5500}}}
(1) {{{8.5*250 + 4.5*750 = 5500}}}
{{{2125 + 3375 = 5500}}}
{{{5500 = 5500}}}
OK