The average can be given by the following formula:
Average = (a1 + a2 + a3 + a4 + a5) / (5).
Equivalently.
Average = (ai) / (i).
Where,
i = 1,2,3,4, ..., n
Substituting
Average = (90 + 85 + 80 + 75 + 75) / (5).
Average = 81
answer:
Average = (a1 + a2 + a3 + a4 + a5) / (5).
or
Average = (ai) / (i).
Where,
i = 1,2,3,4, ..., n
= 
The coefficients are 8 , 12, 6, 1
= 
The coefficients are 16, 96, 216, 216, 81
=
The coefficients are 27, 54,36 , 8
= x^4+4x^3y+6x^2y^2+4xy^3+y^4
The coefficients are 1, 4, 6, 4, 1
each sister gets 21 dollars, so there are three sisters there fore 21*3 equals 63