You can 
put this solution on YOUR website! 
John's rate is 1/18 task per day
Peter's rate is 1/12 task per day
We divide 1 task by the sum of their daily rates of work to get the number of days required for both working together:
 = 7.2 days  (7 days, 4 hours, 48 minutes)
 = 7.2 days  (7 days, 4 hours, 48 minutes)