Question 316085
(1)
Let {{{a}}} = ml of 5% alcohol needed
Let {{{b}}} = ml of 10% alcohol needed
given:
(1) {{{a + b = 2000}}} ml
{{{(.05a + .1b)/2000 = .06}}}
{{{.05a + .1b = 120}}}
(2) {{{5a + 10b = 12000}}}
Multiply both sides of (1) by {{{5}}} and 
subtract from (2)
(2) {{{5a + 10b = 12000}}}
(1) {{{-5a - 5b = -10000}}}
{{{5b = 2000}}}
{{{b = 400}}}
and, since
(1) {{{a + b = 2000}}}
{{{a + 400 = 2000}}}
{{{a = 1600}}}
1600 ml of 5% alcohol and 400 ml of 10% alcohol must be mixed