It would take 2 hours. To complete the whole lawn you would need:

To make a whole, so to get that we multiply:

So we use the same number required for the whole lawn, and multiply it times the hours
I wish I can help but I’m bad at this sorry!
Answer: 2/13
Step-by-step explanation:
4 aces, 4 nines in a deck of 52 cards
so 8/52 chance of getting one of them reduce to 2/13
Two divided by three is 0.6666 repeating.