Question 165151
Let {{{a}}}= amount invested @ 8%
Let {{{b}}}= amount invested @ 6%
Let {{{c}}}= amount invested @ 4%
It is given that
(1) {{{a + b + c = 25000}}}
(2) {{{.08a + .06b + .04c = 1520}}} annually
(3) {{{a = 2b}}}
This is 3 equations and 3 unknowns, so it should
be solvable
Multiply both sides of (2) by {{{100}}}
(2) {{{8a + 6b + 4c = 152000}}}
Multiply both sides of (1) by {{{4}}} and 
subtract from (2)
(1) {{{4a + 4b + 4c = 100000}}}
(4) {{{4a + 2b = 52000}}}
Since {{{a = 2b}}}
(4) {{{4*(2b) + 2b = 52000}}}
(4) {{{10b = 52000}}}
{{{b = 5200}}}
{{{a = 2b}}}
{{{a = 10400}}}
and
(1) {{{a + b + c = 25000}}}
(1) {{{10400 + 5200 + c = 25000}}}
{{{c = 25000 - 15600}}}
{{{c = 9400}}}
$10,400 was invested @ 8%
$5,200 was invested @ 6%
$9,400 was invested @ 4%
check:
(1) {{{a + b + c = 25000}}}
(1) {{{10400 + 5200 + 9400 = 25000}}}
{{{25000 = 25000}}}
(2) {{{.08a + .06b + .04c = 1520}}}
(2) {{{.08*10400 + .06*5200 + .04*9400 = 1520}}}
(2) {{{832 + 312 + 376 = 1520}}}
{{{1520 = 1520}}}
(3) {{{a = 2b}}}
{3) {{{10400 = 2*5200}}}
(3) {{{10400 = 10400}}}
OK