4/5 = ?

8/10 = 12/15 = 16/20 =...

It's already a fraction so I don't understand that.

As for the percentage, I don't know how you are required to do this or what kind of class it is, but one way is to set it up as a ratio. A percentage is how many out of 100. So you can do:

\frac45\,=\,\frac{?}{100}

And then you can cross-multiply.

The number you have mentioned is already in fraction. Ths simplest way to get percentage is to multiply the fraction by 100 (percent is per 100), for example 4/5=0.8 and multiplying by 100 you get 80%. I think this will be clear for you.