Question 686125
I have to take it slow and step-by-step.
Let {{{ p }}} = the number of pages in her book 
She read 5/8 of the pages in her book
So there are 3/8 of the pages left.
Now she reads 18 pages, so she has
3/8 of the pages - 18 pages
OK, so far this is wha she has left to read:
{{{ ( 1 - 5/8 )*p - 18 }}} and now
{{{ ( 1 - 5/8)*p - 18 = (1/3)*p }}}
{{{ (3/8)*p - 18 = (1/3)*p }}}
Multiply both sides by {{{ 24 }}}
{{{ 9p - 432 = 8p }}}
{{{ p = 432 }}}
There are 432 pages in her book
check:
She read {{{ (5/8)*432 = 270 }}}
{{{ 432 - 270 = 162 }}} pages left
{{{ 162 - 18 = 144 }}}
and 
{{{ (1/3)*432 = 144 }}}
OK