Answer:
12P3 = 1320
Step-by-step explanation:
How you need select three persons of a group of twelve persons, but in the selection each member of the group has a different position, then you need use a permutation
Permutation is defined how

In our case n = 12 and r = 3
Thus

12P3 = 1320
81÷s = 90÷100
s= 81 x 100 ÷ 90
s= 90
81 students is 90% of 90 students
B 3.75 it is simple it is easy