Question 791384
Let {{{ a }{}} = total number of stamps bought
Let {{{ b }}} = number of 17 cent stamps bought
Let {{{ c }}} = number of 25 cent stamps bought
Let {{{ d }}} = number of 2 cent stamps bought
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given:
(1) {{{ a < 840 }}} ( in cents )
(2) {{{ b = 3c }}}
(3) {{{ d = 4c }}}
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The number of 17 cent stamps was {{{ 3c }}}, so I
can say {{{ (3c)*17 + 25c < 840 }}}
{{{ 51c + 25c < 840 }}}
{{{ c < 840 / 76 }}}
{{{ c < 11 }}} and a fraction
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Now I can include the 2 cent stamps
{{{ (4c)*2 + c*25 + (3c)*17 < 840 }}}
{{{ 8c + 25c + 51c < 840 }}}
{{{ 84c < 840 }}}
{{{ c < 10 }}} 
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So, the most 25 cent stamps is 9
check answer:
{{{ 9*25 = 225 }}}
{{{ 225 + 3*9*17 + 4*9*2 < 840 }}}
{{{ 225 + 459 + 72 < 840 }}}
{{{ 756 < 840 }}}
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See if this still hold if there is 1 more 25 cent stamp
{{{ 10*25 + 3*10*17 + 4*10*2 < 840 }}}
{{{ 250 + 510 + 80 < 840 }}}
{{{ 840 < 840 }}}
Not true, so answer is correct