Question 1140590
Let {{{ x }}} = total number at the concert
{{{ .2*.75x = .15x }}} is the number of boys
{{{ .2*.25x = .05x }}} is the number of girls
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{{{ .15x = .05x + 36 }}}
{{{ .1x = 36 }}}
{{{ x = 360 }}}
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(a)
{{{ .15*360 = 54 }}} is the number of boys at concert
(b)
Let {{{ n }}} = number of boys who left concert
{{{ .1*( x - n ) = 54 - n }}}
{{{ .1*( 360 - n ) = 54 - n }}}
{{{ 36 - .1n = 54 - n }}}
{{{ .9n = 54 - 36 }}}
{{{ .9n = 18 }}}
{{{ n = 20 }}}
20 boys left the concert
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check:
{{{ 360 - 20 = 340 }}} people remain after 20 boys left
{{{ .1*340 = 34 }}} boys remain after 20  boys left
{{{ 34 + 20 = 54 }}} is number of boys starting out
OK