Question 826447
Let {{{ a }}} = the cost for 1 tablet in cents
Let {{{ b }}}  = the cost of 1 pen in cents
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(1) {{{ 7a + 4b = 640 }}}
(2) {{{ 2a + 19b = 540 }}} ( in cents )
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Multiply both sides of (1) by {{{2}}}
Multiply both sides of (2) by {{{ 7 }}}
Subtract (1) from (2)
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(2) {{{ 14a + 133b = 3780 }}}
(1) {{{ -14a - 8b = -1280 }}}
{{{ 125b = 2500 }}}
{{{ b = 20 }}}
and, since
(1) {{{ 7a + 4*20 = 640 }}}
(1) {{{ 7a = 640 - 80 }}}
(1) {{{ 7a = 560 }}}
(1) {{{ a = 80 }}}
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The price of a tablet is 80 cents
The price of a pen is 20 cents
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check:
(2) {{{ 2*80 + 19*20 = 540 }}} 
(2) {{{ 160 + 380 = 540 }}}
(2) {{{ 540 = 540 }}}
OK