Going 20mph the whole time would be faster by 12 minutes.
We can tell this by determining the length of time both will take. First let's take the easier one, which is the 20mph constantly. If he can travel 20 miles in 1 hour, finding out how long it will take him to go 60 miles is as simple as dividing.
60 miles/20 miles per hour = 3hours.
Now the process is slightly different for the first way. First we have to see how far he traveled in the first two hours. Since he went 10mph and for 2 hours, this is a simple multiplication problem.
2 hours * 10 miles per hour = 20 miles
Now we know he still has 40 miles to travel and can move at 30mph. We can repeat the process from step one to see how long this will take.
40 miles/ 30 miles per hour = 1.2 hours.
This shows us that the first way will take 3.2 hours (or 3 hours and 12 minutes), whereas the second way will only take 3 hours.
