Answer:
<em>Grades to be scored in 3rd exam = 88</em>
Step-by-step explanation:
Given that average of 3 numbers is 85.
First two numbers are 81 and 86.
To find:
3rd number so that average is 85
Solution:
Let the third number = 
Formula for average is:

Here, sum of numbers = 
Total count of numbers = 3

<em>Grades to be scored in 3rd exam = 88</em>
Rational....because it can be written as a ratio
The answer is 0
The formula is
n!/ r!(n-r)!
4!/ 9!(4-9)!
It would be 56/177. If you need any question how I got that answer, ask me.. :)