How many 20 milligram Adderal pills should I take?

I have an exam coming up for my math cl and REALLY need to p. I don't get the material at all but know that I need to study, a lot. I have four 20 milligram adderalls and am wondering how many I should take? Also, when should I take them? I hear it doesn't kick in til 2-3 hours after ingesting.