Question 1029987
Here is my approach - others may have a different method. When I think of rate, it's "miles per hour" or "units per time" 

Vince can mow 6-2/3 lawns per hour. (60/9 = 6.667)

Vince + Phil mow 12 lawns per hour. (60/5 = 2)

So it's simple subtraction, Phil can mow 5-1/3 lawns in 1 hour. And 60/5.33= 11.25 or 11-1/4 minutes to mow one.