Matthew Clemens, Physics and Math Teacher, Parent, Tutor, and Professional Ski Instructor
The way you worded your question (or at least the units you used for one of your numbers) makes what you are trying to find unclear. Units of m/s are units of speed (or how fast?) already. I assume you meant "How fast would a car HAVE TO be going if IT TRAVELED 400 METERS in 20 seconds?" If this is indeed what you intended to ask, we could find the AVERAGE SPEED needed to travel 400m in 20s by using the formula: average speed = distance traveled / elapsed time = 400m/20s = 20m/s. However, this is an average speed so the car could have been moving faster or slower at different points during those 20 seconds.