Question 1040269
This is telling me that plan B is the better choice
for a certain number of minutes, but after that, 
plan A is better. 
Let {{{ n }}} = number of minutes of calls
with plan B
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Set up an equation that makes the 2 plans equal
and then add 1 more minutes to {{{ n }}}
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plan A = plan B
{{{ 18.5 = 6.5 + .1*( n - 40 ) }}}
{{{ 18.5 = 6.5 + .1n - 4 }}}
{{{ 12 = .1n - 4 }}}
{{{ .1n = 16 }}}
{{{ n = 160 }}}
So this says that after the plans cost the same ( $18.50 )
per month with both using 160 minutes
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Now let {{{ n = 161 }}} and solve the right side
{{{ 6.5 + .1*( 161 - 40 ) }}}
{{{ 6.5 + 16.1 - 4 }}}
{{{ 16.1 + 2.5 = 18.6 }}}
So, at 161 minutes, plan B becomes more expensive
than plan A ( plan B costs $18.60 )