Question 499925
{{{ 15 = 18 - d*18 }}}
where {{{d}}} is the discount as a fraction 
Add {{{ d*18 }}} to both sides
{{{ 15 + d*18 = 18 }}}
{{{ d*18 = 18 - 15 }}}
{{{ d = 3/18 }}}
{{{ d = 1/6 }}}