Question

find the mean of the prime number between 70 and 100​

Answer 1

First, reject the obvious non-prime numbers. None of the even numbers can be a prime because they can be divided by two. None of the numbers whose last digit is 5 is prime because it divides by 5. Every third odd number (check them of course) can be divided by 3 (see Worksheet 2). So: for the odd numbers between 70 and 100:

75, 85, 95 are divisible by 5

75, 81, 87, 93, 99 are divisible by 3

This leaves 71, 73, 77, 79, 83, 89, 91, 97. Every 7th odd number divides by 7 - thesenumbersare77and91(77=7×11and91=7×13.) Thenextprime number is 11, but 97÷ 11 = 8 plus remainder 9, i.e. the quotient is smaller than 11. So there are no more prime number factors. The prime numbers between 70 and 100 are 71, 73, 79, 83, 89, 97

Answer 2

Answer:

The prime numbers between 70 and 100 are 71, 73, 79, 83, 89, 97.

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