Question 804278
{{{ 105*(x/(x+70))+45*(x/(x+70))=100 }}}
You can multiply both sides by {{{ ( x+70 )/x }}}
with the provision that {{{ x }}} cannot  = {{{ 0 }}}
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{{{ 105 + 45 = (( x+70 )/x)*100 }}}
{{{ 150 = (( x+70 )/x)*100 }}}
Divide both sides by {{{ 100 }}}
{{{ 1.5 = ( x+70 )/x }}}
Multiply both sides by {{{ x }}}
{{{ 1.5x = x + 70 }}}
{{{ .5x = 70 }}}
{{{ x = 140 }}}
check answer:
{{{ 105*(x/(x+70))+45*(x/(x+70))=100 }}}
{{{ 105*(140/(140+70))+45*(140/(140+70))=100 }}}
{{{ 105*(2/3) + 45*(2/3) = 100 }}}
{{{ 35*2 +15*2 = 100 }}}
{{{ 70 + 30 = 100 }}}
{{{ 100 = 100 }}}
OK