Unit Rate
Tom runs 12 miles in 3 hours. We can calculate two different unit rates:
12 miles / 3 hours = 4 miles/hour
3 hours / 12 miles = 0.25 hours/mile
The first rate measures how many miles Tom runs per hour. The second rate measures how long it takes to Tom to run one mile.
We are asked to find the time needed for Tom to run 20 miles, thus the second rate will be useful:
0.25 hours/mile * 20 miles = 5 hours
To run 20 miles, Tom will take 5 hours