Question 1060865
I do this the " long way " which might
be easier to understand
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Let {{{ n }}} = number of nickels 
Let {{{ d }}} = number of dimes
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(1) {{{ n + d = 90 }}}
(2) {{{ 5n + 10d = 640 }}} ( in cents )
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Multiply both sides of (1) by {{{ 5 }}} and
subtract (1) from (2)
(2) {{{ 5n + 10d = 640 }}} 
(1) {{{ -5n - 5d = -450 }}}
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{{{ 5d = 190 }}}
{{{ d = 38 }}}
and
(1) {{{ n + d = 90 }}}
(1) {{{ n + 38 = 90 }}}
(1) {{{ n = 52 }}}
There are 52 nickels and 38 dimes
( same as their answer, since {{{ X }}} is number of nickels )
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check answer:
(2) {{{ 5n + 10d = 640 }}} 
(2) {{{ 5*52 + 10*38 = 640 }}} 
(2) {{{ 260 + 380 = 640 }}}
(2) {{{ 640 = 640 }}}
OK