Question 1148592
You are given 2 points on the straight line
( 36, 17.53 )
( 75, 21.82 )
Let {{{ C }}} = Monthly cost of plan
Let {{{ t }}} = calling time in minutes 
You can use the general point-slope formula
{{{ ( C - 17.53 ) / (  t - 36 ) = ( 21.82 - 17.53 ) / ( 75 - 36 ) }}}
{{{ ( C - 17.53 ) / (  t - 36 ) = 4.29 / 39 }}}
{{{ ( C - 17.53 ) / (  t - 36 ) = .11 }}}
{{{ C - 17.53 = .11t - 3.96 }}}
{{{ C = .11t + 13.57 }}}
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{{{ t = 59 }}} min
{{{ C = .11*59 + 13.57 }}}
{{{ C = 6.49 + 13.57 }}}
{{{ C = 20.06 }}}
$20.06 per month
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