# What on Earth is Average Speed?

Vivian Kerr is a regular contributor to several GMAT and SAT websites, allowing her to flex her intellectual muscle while she is in between film and stage project as an actress.

SAT word problems scare you? Me too! Especially once they start talking about distance, work rates, trains leaving stations, etc. Sometimes the SAT likes to throw a particularly challenging word problem towards the end of a section, and this type of question can often involve a concept called “average speed.” Well, I know what “average” means, so this should be easy, right? Unfortunately, “average speed” has nothing to do with the mathematical average, or the mean. This is the formula to know: Average Speed = Total Distance / Total Time.

If you simply try to apply the “average” formula (take the mean) in these questions, you’ll get them wrong! Let’s say we see a question that goes something like this:

#1. Suzi drove 20 miles to the grocery store going 10 mph. Then she drove 24 miles to the movies to see Skyfall with a friend but only went 8 mph due to heavy traffic. What was her average speed for the whole trip?

(A) 8.4mph
(B) 8.8mph
(C) 9.0mph
(D) 9.6mph
(E) 10.0mph

A student who forgets that “average speed” is different from the “average” will probably try to solve by adding the speeds and dividing by 2. 10 + 8 / 2 = 18/2 = 9.

The savvy student plugs in the “average speed” formula. Total distance = 20 + 24 = 44. Total time = 2 hours + 3 hours = 5. The distance divided by the time = 44/5 = 8.8.