Question 581167
Let {{{ d }}} = number of 10 cent coins
Let {{{ n }}} = number of 5 cent coins
given:
(1) {{{ d + n = 16 }}}
(2) {{{ 10d + 5n = 100 }}} ( in cents )
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Multiply both sides of (1) by {{{ 5 }}}
and subtract (1) from (2)
(2) {{{ 10d + 5n = 100 }}} 
(1) {{{ -5d - 5n = -80 }}}
{{{ 5d = 20 }}}
{{{ d = 4 }}}
and
(1) {{{ d + n = 16 }}}
(1) {{{ 4 + n = 16 }}}
(1) {{{ n = 12 }}}
The number of 10 cent coins is 4
The number of 5 cent coins is 12