Question 1095931
b, book price
p, pen price

{{{system(2b+7p=79,7b+5p=77)}}}

Multiply first equation by 7;
Multiply second equation by 2:


{{{system(14b+49p=553,14b+10p=154)}}}
Subtract this second of these from the first of these.


{{{39p=399}}}


{{{p=10.23}}}


From {{{2b+7p=79}}}, 

solving for b gives {{{b=(79-7*p)/2}}}
{{{b=(79-7*10.23)/2}}}

{{{b=3.69}}}


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One book, {{{highlight(b=3.69)}}}
One pen, {{{highlight(p=10.23)}}}
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Sum for 1 book and 2 pens:
{{{3.69+2*(10.23)}}}

{{{highlight(highlight(24.15))}}},{{{rs}}}