I have some notes here from eduplace.com that might help you answering the problem:
"When rates are expressed as a quantity of 1, such as 2 feet per second or 5 miles per hour, they are called unit rates. If you have a multiple-unit rate such as 120 students for every 3 buses, and want to find the single-unit rate, write a ratio equal to the multiple-unit rate with 1 as the second term."
I hope my answer has come to your help. God bless and have a nice day ahead!
