Question 455082
Let {{{a}}} = gallons of 60% solution needed
given:
{{{ .6a }}} = gallons of salt in 60% solution
{{{ .13*30 = 3.9 }}} = gallons of salt in 60% solution
{{{ (.6a + 3.9) / ( a + 30) = .5 }}}
{{{ .6a + 3.9 = .5*( a + 30 ) }}}
{{{ .6a + 3.9 = .5a + 15 }}}
{{{ .1a = 11.1 }}}
{{{ a = 111 }}}
111 gallons of 60% solution are needed 
check answer:
{{{ (.6a + 3.9) / ( a + 30) = .5 }}}
{{{ (.6*111 + 3.9) / ( 111 + 30) = .5 }}}
{{{ ( 66.6 + 3.9) / 141 = .5 }}}
{{{ 70.5 / 141 = .5 }}}
{{{ 70.5 = .5*141 }}}
{{{ 70.5 = 70.5 }}}
OK