Question 1205771
<pre>
{{{highlight(cross(a))}}} {{{highlight(A)}}} book shelf {{{highlight(cross(contain))}}} {{{highlight(contains)}}} x {{{highlight(cross(book))}}} {{{highlight(books)}}}. When 4/5 of the
{{{highlight(cross(book))}}} {{{highlight(books)}}} are removed, {{{highlight(cross(i))}}} {{{highlight(I)}}} have 25 {{{highlight(cross(book))}}} {{{highlight(books)}}} left. {{{highlight(cross(how))}}} {{{highlight(How)}}} many
{{{highlight(cross(book))}}} {{{highlight(books)}}} were on the {{{highlight(cross(shelf))}}} {{{highlight("shelf?")}}}

{{{(matrix(2,1,x,books))-expr(4/5)(matrix(2,1,x,books))}}}{{{""=""}}}{{{(matrix(2,1,25,books))}}}

{{{x-expr(4/5)x = 25}}}

Multiply through by 5 to clear the fraction:

{{{5x-4x=125}}}

{{{x=125}}} books.

Edwin</pre>