To find the percent (p) you use the Total number of problems (T) and the number correct (C) and the next formula:
Then:
Noelle:
T=25
C=18
p=72%
Bruce:
T=20
C=14
p=70%
Sean:
T=30
C=24
p=80%
Leia:
T=40
C=31
p=77.5%
-----------------------
Then, from worst score to best score:Bruce (70%), Noelle (72%), Leia (77.5%) and Sean (80%)