My way is probably not the correct way but let's go with it.
If you take 9 am and add 4 hours, it would be 1 pm. If you add 6, it would be 3 pm. Then starting back at 1 pm for the cycle to repeat. 1 pm + 4 hours would be 5 pm. 3 pm + 6 hours is 9 pm. Then again... 5 pm plus 4 hours would also be 9 pm.
Now they are the same! So it would be 9 pm. I hope this helped.
