Answer:
105
Step-by-step explanation:
The formula for finding average, a.k.a. mean, is the sum of the terms divided by the number of terms. So to find the 4th number, you need to do 85x4 first, to get 340, which is the sum of the terms(specifically 76, 78, 81, and the unknown number). 76+78+81=235. 340-235=105, therefore 105 is the fourth number.
