Imagine that there are \(10\) questions in a test and you get \(7\) of them correct. You would say that you got \(\frac{7}{{10}}\), because \(7\) as a fraction of \(10\) is \(\frac{7}{{10}}\)
In the same way, \(4\) as a fraction of \(12\) is \(\frac{4}{{12}}\) which cancels to \(\frac{1}{{3}}\)
Similarly \(20\) as a fraction of \(48\) is \(\frac{{20}}{{48}}\) which is \(\frac{{5}}{{12}}\)
Seems easy, but just be careful with the units.
For example, \(10p\) as a fraction of \(\pounds20\) is not: \(\frac{{10}}{{20}}\)
It is \(\frac{{10}}{{2000}}\) (because \(\pounds20\) is \(2000p\) and we must have the same units top and bottom).
This cancels to \(\frac{1}{200}\)
Similarly, \(30cm\) as a fraction of \(5m\) is \(\frac{{30}}{{500}}\) (because \(5m = 500cm\)).