Question 190494
Let x=no of $8-book
.   y=no of $3-book
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We know that there were 20 books, so:
{{{x+y=20}}}     (1)
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Also, we know that the cost of all the books is $85, so:
{{{8x+3y=85}}}   (2)
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Let's multiply (1) by 3 to get:
{{{3*(x+y)=20*3}}}
{{{3x+3y=60}}}
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Let's subtract this new equation from (2) to get:
{{{8x+3y=85}}}
-{{{3x+3y=60}}}
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{{{5x=25}}}
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Dividing both sides by 5 we get:
{{{5x/5=25/5}}}
{{{x=5}}}
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Now we know that there were 5 8-dollar books and 20-5=15 3-dollar books.
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To check:
5*8+15*3=?85
40+45=?85
85=85