A is the mother of B.
B is the father of C.
C is the wife of D.
This means that A is the grandmother-in-law of D.
The minimum mark a student could have scored is 20.
92 * 10 people = 920
The amount of times that 100 fits in 920 is 9 times.
This leaves you with 9 people scoring 100 and one person with a score of 20.