Question 493651
Let {{{a}}} = gallons of cream needed
Let {{{b}}} = gallons of milk needed
given:
(1) {{{ a + b = 18 }}}
(2) {{{ .2a + .02b = .04*18 }}}
---------------------
(2) {{{ 20a + 2b = 72 }}}
Multiply both sides of (1) by {{{2}}},
and subtract (1) from (2)
(2) {{{ 20a + 2b = 72 }}}
(1) {{{ -2a - 2b = -36 }}}
{{{ 18a = 36 }}}
{{{ a = 2 }}}
and, since
(1) {{{ a + b = 18 }}}
1) {{{ b = 16 }}}
2 gallons of cream are needed
16 gallons of milk are needed
check:
(2) {{{ .2a + .02b = .04*18 }}}
(2) {{{ .2*2 + .02*16 = .04*18 }}}
(2) {{{ .4 + .32 = .72 }}}
{{{ .72 = .72 }}}
OK