 Okay, let's try an example where we need to change both parts of a compound unit. When Sally Pearson won the 100m hurdles at the 2012 World Championships, she ran at an average speed of 8.10m per second. Let's convert this to km per hour, which is a speed unit that you're probably more familiar with. First, write what you know. With the compound unit arranged appropriately, meters on the top, per means divided by seconds on the bottom. Now, we need to convert the meters into kilometers and the seconds into hours. Now don't fret about this being difficult. It's not. We just treat the two conversions separately. First, let's deal with the meters. Put in the conversion factor for meters to kilometers and cancel out the meters. If we were to do the calculation now, we would end up with a speed that was in kilometers per second because we haven't dealt with the seconds yet. So let's now convert the seconds to hours. Let's go to minutes first. Now, note that the units don't have to be directly next to each other for you to be able to cancel them. They're just part of the numerator or denominator of one big fraction. And now we'll convert the minutes to hours. And there we are, just a fraction under 30 kilometers an hour.