Basic math.
Vp / Vs = Np / Ns
Now substitute the quantities you have:
Vp / 25 = 20 / 100
Get Vp on one side by multiplying both sides by the denominator...
(Vp / 25) * 25 = (20 / 100) * 25
Vp = (20 / 100) * 25 = 5 VAC
So, the primary is 5 volts AC. Transformers can be step up and step down. This one is a step up transformer.
It looks like you need to brush up on your basic algebra before continuing the Basic Electronics course. You can't do much of any electronics without at least a basic (1st year high school) math background.
There are quite a few good books which can help you. Possibly, if you're up front with your teacher that you're a little light on math skills, he can recommend one he's familiar with. He might even recommend a good tutor or other resource to help you out.
This group usually doesn't provide answers to homework questions.
Good luck Chris