Formula Used: Average =( Sum of the terms )∕( Number of the terms ) Calculation: The prime numbers between 70 to 90 are 71,73,79,83 and 89 Average =( Sum of the terms )∕ Number of the terms ) Average =(71+73+79+83+89)∕5 ⇒395∕5=79 ∴ The average of all prime numbers between 70 to 90 is 79.