Question 1138669
Let {{{ a }}} = amount invested in CDs
Let {{{ b }}} = amount invested in bonds
Let {{{ c }}} = amount invested in mortgages
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Use simple interest formula
{{{ I = P*r*t }}}
(1) {{{ a + b + c = 132000 }}}
(2) {{{ I[1] + I[2] + I[3] = 11200 }}}
For CDs:
(3) {{{ I[1] = a*.06*1 }}}
For bonds:
(4) {{{ I[2] = b*.08*1 }}}
For mortgages:
(5) {{{ I[3] = c*.1*1 }}}
and
(6) {{{ a + b = c }}}
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I can substitute (3), (4), and (5) into (2)
(2) {{{ .06a + .08b + .1c = 11200 }}}
(2) {{{ 6a + 8b + 10c = 1120000 }}}
(2) {{{ 3a + 4b + 5c = 560000 }}}
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Subtracting (6) from (1)
 {{{ 2c = 132000 }}}
{{{ c = 66000 }}}
and
(2) {{{ 3a + 4b + 5*66000 = 560000 }}}
(2) {{{ 3a + 4b =  560000 - 330000 }}}
(2) {{{ 3a + 4b = 230000 }}}
Multiply both sides of (1) by {{{2}}} and 
subtract (1) from (2)
(1) {{{ 4a + 4b = 264000 }}}
(2) {{{ -3a - 4b = -230000 }}}
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{{{ a = 34000 }}}
and
(6) {{{ a + b = c }}}
(6) {{{ 34000 + b = 66000 }}}
(6) {{{ b = 32000 }}}
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$34,000 = amount invested in CDs
$32,000 = amount invested in bonds
$66,000 = amount invested in mortgages
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check:
(2) {{{ .06a + .08b + .1c = 11200 }}}
(2) {{{ .06*34000 + .08*32000 + .1*66000 = 11200 }}}
(2) {{{ 2040 + 2560 + 6600 = 11200 }}}
(2) {{{ 11200 = 11200 }}}
OK