Question 411740
Let {{{a}}} = gallons of 90% antifreeze needed
{{{(.9a + .3*90) / (a + 90) = .8}}}
{{{(.9a + 27) / (a + 90) = .8}}}
{{{.9a + 27 = .8*(a + 90)}}}
{{{.9a + 27 = .8a + 72}}}
{{{.1a = 45}}}
{{{a = 450}}}
450 gallons of 90% antifreeze are needed
check:
{{{(.9a + .3*90) / (a + 90) = .8}}}
{{{(.9*450 + .3*90) / (450 + 90) = .8}}}
{{{(405 + 27) / 540 = .8}}}
{{{432/540 = .8}}}
{{{432 = .8*540}}}
{{{432 = 432}}}
OK