I'm trying to figure out question 4 but I don't understand it and neither do my friends.

Step-by-step explanation:

You know the relation between X and Z is ...

  Z = (X -μ)/σ

Solving for X, we find ...

  Zσ +μ = X

You're trying to find the exam scores that correspond to the Z-values you found in question 3.

  A = 90th percentile and above = 1.28(8) +78 = 88.24 ≈ 88

  B = 80th percentile and above = 0.84(8) +78 = 84.72 ≈ 85

  C = 70th percentile and above = 0.52(8) +78 = 82.16 ≈ 82

  D = 60th percentile and above = 0.25(8) +78 = 80

So, your blanks are filled like this ...

  A: 88 — 100

  B: 85 — 87

  C: 82 — 84

  D: 80 — 81

  F: 0 — 79

Comment on this curve

The setup guarantees that 60% of the class will fail. It is more common to have those below Z = -2 get an F, and those above Z = 2 get an A.