how would i calculate these probabilities, if i'm not given success...?

the problem is as follows: "a student didn't study for a 20 question quiz which has 5 possible answers, and therefore will guess on every question. find each probability-prob. the student gets at least 50% on test, prob. student gets at most 75% on test, and prob. student gets at least a 70% on the test."

well, i know prob right=1/5 and prob wrong= 4/5. so, when setting up my prob. for each part, would pi be .2 because that's success right?