Question 1206099

 

let psychology textbooks be {{{p}}} and history textbooks {{{h}}}

if the store sold a combined total of {{{215}}}, we have 

{{{p+h=215}}}....eq.1

if the number of psychology textbooks sold was {{{57}}} more than the number of history textbooks sold, we have 

 {{{p=h+57}}}...eq.2

substitute  {{{p}}} in eq.1

{{{h+57+h=215}}}....eq.1

{{{2h=215-57}}}

{{{2h=158}}}

{{{h=79}}}

go to eq.2, substitute {{{h}}}

{{{p=79+57}}}...eq.2

{{{p=136}}}

so, {{{136}}} psychology and {{{79}}} history textbooks were sold