A track runner ran exactly 400 meters in 45.23 seconds. what was the average speed in miles per hour?

First we will convert meters to miles and then find the speed of the runner in miles per hour.

400 meters is 0.4 kilometers (1000 meters in 1 kilometer, so 400m / 1000m)

Now let's convert 0.4 kilometers into miles.

We do this by using the kilometer to miles conversion rate which is 0.621371. (This means that ##"1 kilometer" = "0.621371 miles"##)

So ##"0.4 km" * 0.621371 "miles"/"km" = "0.2485484 miles"##

45.23 seconds is 0.012... hours (there are 3600 seconds in 1 hour, so 45.23s / 3600s/h)

So our speed is 19.782760114967941631660402387796 miles per hour.

Using sig figs we get an answer of 19.78 miles per hour. That is one very fast runner!