Question 1158483
Let {{{ a }}} = number of 5c coins
Let {{{ b }}} = number of 20c coins
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(1) {{{ a + b = 30 }}}
(2) {{{ 5a + 20b = 345 }}} ( in cents )
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(2) {{{ a + 4b = 69 }}}
Subtract (1) from (2)
(1) {{{ -a - b = -30 }}}
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{{{ 3b = 39 }}}
{{{ b = 13 }}}
and
(1) {{{ a + 13 = 30 }}}
(1) {{{ a = 17 }}}
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number of 5c coins = 17
number of 20c coins = 13
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check:
(2) {{{ 5a + 20b = 345 }}} 
(2) {{{ 5*17 + 20*13 = 345 }}} 
(2) {{{ 85 + 260 = 345 }}}
(2) {{{ 345 = 345 }}}
OK