Did you know Google can do maths? It's got a built in calculator that understands minutes and seconds.
Let's say you know it takes you five minutes to run one kilometre. Obviously in 30 minutes you would run 6 kilometres (30 divided by 5). But what if it takes you five minutes and fourteen seconds to run a kilometre? Then how far would you run in 30 minutes?
You can ask Google. Just type in '30 minutes divided by 5 minutes 14 seconds' and click the search button.
You can do it in reverse, too. How long would it take to run 5 kilometres at a speed of 7 minutes 15 seconds per kilometre? Go to Google and enter '7 minutes 15 seconds times 5'.