Question 227475
Let {{{a}}}= liters of 20% solution needed
Let {{{b}}}= liters of 70% solution needed
{{{(.2a + .7b)/5 = .4}}}
{{{.2a + .7b = 2}}}
(1) {{{2a + 7b = 20}}}
(2) {{{a + b = 5}}}
Multiply both sides of (2) by {{{2}}}
and subtract from (1)
(1) {{{2a + 7b = 20}}}
(2) {{{-2a + -2b = -10}}}
{{{5b = 10}}}
{{{b = 2}}}
and, since
{{{a + b = 5}}}
{{{a = 5 - b}}}
{{{a = 5 - 2}}}
{{{a = 3}}}
2 liters of 20% solution and 3 liters of 70% 
solution are needed