Question 917307
DEFINING A VARIABLE TO START:
{{{x}}}= number of books sold at Rs. 18 each, which makes
{{{120-x}}}= number of books sold at Rs. 12 each.
  
REASONING:
So,
{{{18x}}}= money (in Rs.) received from the sale of {{{x}}} books at Rs. 18 each;
{{{12(120-x)}}}= money (in Rs.) received from the sale of {{{120-x}}} books at Rs. 12 each,
and {{{18x+12(120-x)=1920}}}= total receipts (in Rs.).
The equation to solve is {{{18x+12(120-x)=1920}}}
 
SOLVING THE EQUATION:
{{{18x+12(120-x)=1920}}}
{{{18x+12*120-12x=1920}}}
{{{18x+1440-12x=1920}}}
{{{18x-12x+1440=1920}}}
{{{(18-12)x+1440=1920}}}
{{{6x+1440=1920}}}
{{{6x=1920-1440}}}
{{{6x=480}}}
{{{x=480/6}}}
{{{highlight(x=80)}}}
{{{120-x=120-80}}}
{{{highlight(120-x=40)}}}
{{{highlight(80)}}} volumes were sold at Rs. 18 each,
and the remaining {{{highlight(40)}}} volumes were sold at Rs. 12 each.