Question 221223
Let {{{a}}} = tons of $125/ton hay used
Let {{{b}}} = tons of $75/ton hay used
given:
(1) {{{a + b = 100}}}
(2) {{{125a + 75b = 9000}}}
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Multiply both sides of (1) by {{{75}}} and
subtract from (2)
(2) {{{125a + 75b = 9000}}}
(1) {{{-75a + -75b = -7500}}}
{{{50a = 1500}}}
{{{a = 30}}}
and, since 
{{{a + b = 100}}}
{{{30 + b = 100}}}
{{{b = 70}}}
30 tons of $125/ton hay and 70 tons of $75/ton hay
must be purchased
check:
(2) {{{125a + 75b = 9000}}}
{{{125*30 + 75*70 = 9000}}}
{{{3750 + 5250 = 9000}}}
{{{9000 = 9000}}}
OK