Question 1197758
 The corresponding modeling system is 

{{{x+y=135}}}.......eq..1
{{{x=4y+15}}}......eq.2
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{{{x+y=135}}}.......eq..1, substitute  {{{x}}} from eq.2

{{{4y+15+y=135}}}.....solve for {{{y}}}

{{{5y=135-15}}}

{{{5y=120}}}

{{{y=24}}}

go to

{{{x=4y+15}}}......eq.2, substitute  {{{y}}} value


{{{x=4*24+15}}}

{{{x=111}}}


${{{111}}}= the cost of a calculator and ${{{24}}}= the cost of the textbook