Question 524079
Let {{{C}}} = the total charge
Let {{{h}}} = number of hours 
The equation for PP is:
{{{ C = 300 + 20h }}}
The equation for CC is:
{{{ C = 500 + 12h }}}
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Set the equations equal to eachother
{{{ 300 + 20h = 500 + 12h }}}
{{{ 300 + 8h = 500 }}}
{{{ 8h = 500 - 300 }}}
{{{ 8h = 200 }}}
{{{ h = 200/8 }}}
{{{ h = 25 }}}
After 25 hours, the charges will be the same