Question 981639
Let {{{ x }}} = number of girls in class 
{{{ 100 - x }}} = numbr of boys in class 
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given:
(1) {{{ ( 1/4 )*x = 3*( 1/8 )*( 100 - x ) }}}
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Multiply both sides by {{{ 8 }}}
(1) {{{ 2x = 3*( 100 - x ) }}}
(1) {{{ 2x = 300 - 3x }}}
(1) {{{ 5x = 300 }}}
(1) {{{ x = 60 }}}
and
{{{ 100 - x = 100 - 60 }}}
{{{ 100 - x = 40 }}}
There are 40 boys in the class
{{{ ( 1/8 )*40 = 5 }}}
5 boys play piano
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check:
Three times as many girls play piano, so
{{{ 3*5 = 15 }}} girls play piano
This is 1/4 of the girls, so there are
{{{ 4*15 = 60 }}} girls in the class
OK