Hi.
We know that Earl can run 50 meters in 20 seconds. That means he can run 150 meters in one minute and 9,000 meters (or 9 kilometers) in one hour.
But we're worried about how many meters he can run in one second, not one hour! So what we have to do is set up a ratio:
50 meters / 20 seconds = ? meters / 1 second
Once we have that done, we can cross multiply and solve for the unknown quantity (here, the number of meters he runs in one second). We're going to use the letter "x" to stand in for the number of meters he runs in one second:
50 meters / 20 seconds = x / 1 second
(50 x 1) = (20) (x)
50 = 20x divide both sides by 20 to get the "x" alone
50 / 20 = x
2.5 = x
Earl can run 2.5 meters in one second.
