Question 855078
Let {{{ y }}} = the number of cards he has
at the beginning of week {{{ x }}}
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week 0:
{{{ y = 0 + 9 }}}
week 1:
{{{ y = 4 + 9 }}}
week 2:
{{{ y = 8 + 9 }}}
week 3:
{{{ y = 12 + 9 }}}
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The equation is:
{{{ y = 4x + 9 }}}
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They want to know what {{{ x }}} is ( the week number )
when {{{ y = 57 }}}
{{{ 57 = 4x + 9 }}}
Subtract {{{ 9 }}} from both sides
{{{ 4x = 48 }}}
{{{ x = 12 }}}
At the end of week 12 he has 57 cards
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Here's the plot:
{{{ graph( 400, 400, -3, 15, -10, 70, 4x + 9 ) }}}