Question 764747
payment          change
                                       ${{{1 }}}                    ${{{9}}}
                                       ${{{2 }}}                    ${{{8}}}
                                       ${{{3}}}                     ${{{7}}}

if payment represents {{{x}}} and change {{{y}}}, we have points ({{{1}}},{{{9}}}),({{{2}}},{{{8}}}),({{{3}}},{{{7}}}) that should lie on same line; so, we can fine a line passing through two points:


*[invoke change_this_name10094 1, 9, 2, 8]

so, equation is {{{y=-x+10}}}

now we can check if all three points lie on this line:

{{{drawing(600,600,-10,10,-5,15,grid(0),locate(3,7,p2(3,7)),circle(3,7,0.1),locate(2,8,p1(2,8)),circle(2,8,0.1),locate(1,9,p(1,9)),circle(1,9,0.1),graph(600,600,-10,10,-5,15,-x+10))}}}