Solve
r= s/t, for t
a= 2bc, for c

   r = s/t   Divide both sides by s
r/s = t   Switch the sides to make it easier to read
   t = r/s

     a = 2bc   Divide both sides by 2b
a/2b = c       Switch the sides to make it easier to read
     c = a/2b

You can use a calculater and divide the minutes by the $$, hope that you