It´s a common thing that our students struggle where they have to change units using conversion factors. A CONVERSION FACTOR is a fraction with different units on the numerator and denominator, which are equivalent. We use it to convert a measurement in one set of units to the same measurement in another set of units. For example, from km/h to m/s.
There are many different measurements that sometimes require conversions: length, volume, mass, speed, density... Conversion factors are used for conversion within the International System of Units (SI).
The FIRST step is to bear in mind the conversion you need. For example, in time, you must know that one day has 24 hours, one hour has 60 minutes and one minute has 60 seconds.
Here you have an example, where you need to convert 5000 seconds into hours. So, you must know that 1 hour is equal to 3600 seconds to make the conversion factor:
Another kind of conversion factor is a combined one where you need two fractions to convert two units, like in speed or density. For instance, we must convert 90 km/h and express them in the SI of units, m/s. The equivalence for distance, from km to m, is 1km = 1000 m. The equivalence of time, is the same, 1 hour = 3600 seconds.
We can find another explanation in this video, with more examples: