Question 550032
Let {{{t}}} = time of calls in minutes
Let {{{C}}} = cost of plan
Let the period of each plan = 1 month
1st plan:
{{{ C = 22 + .13t }}}
2nd plan:
{{{ C = 8 + .18t }}}
---------------
Set the costs equal to each other
{{{ 22 + .13t = 8 + .18t }}}
{{{ .18t - .13t = 22 - 8 }}}
{{{ .05t = 14 }}}
{{{ t = 280 }}} 
When call minutes = 280, the cost of the plans are equal
check answer:
{{{ C = 22 + .13*280 }}}
{{{ C = 22 + 36.4 }}}
{{{ C = 58.4 }}}
and
{{{ C = 8 + .18*280 }}}
{{{ C = 8 + 50.4 }}}
{{{ C = 58.4 }}}
OK