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Question

The average of all prime numbers between 60 to 80 is:

This question was previously asked in
UPSSSC PET 2022 Question Paper (16-Oct-2022) (Shift 2)
The correct answer is 70.2

Prime Numbers Average Calculation: 60 to 80

This guide provides a clear, step-by-step method to determine the average of all prime numbers found within the range of 60 to 80.

Prime Number Definition

A prime number is a whole number greater than 1 that has only two divisors: 1 and itself. Numbers like 2, 3, 5, 7, 11 are examples of prime numbers.

Prime Numbers in Range 60-80 Identification

First, let's list all the integers from 60 to 80:

61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79.

Now, we identify which of these numbers are prime:

  • 61: Is prime (divisors: 1, 61).
  • 62: Is not prime (divisible by 2).
  • 63: Is not prime (divisible by 3, 7).
  • 64: Is not prime (divisible by 2).
  • 65: Is not prime (divisible by 5).
  • 66: Is not prime (divisible by 2).
  • 67: Is prime (divisors: 1, 67).
  • 68: Is not prime (divisible by 2).
  • 69: Is not prime (divisible by 3).
  • 70: Is not prime (divisible by 2).
  • 71: Is prime (divisors: 1, 71).
  • 72: Is not prime (divisible by 2).
  • 73: Is prime (divisors: 1, 73).
  • 74: Is not prime (divisible by 2).
  • 75: Is not prime (divisible by 3, 5).
  • 76: Is not prime (divisible by 2).
  • 77: Is not prime (divisible by 7, 11).
  • 78: Is not prime (divisible by 2).
  • 79: Is prime (divisors: 1, 79).

The prime numbers identified between 60 and 80 are: 61, 67, 71, 73, and 79.

Average Calculation Steps

The average (or mean) is found by summing all the numbers and dividing by how many numbers there are.

Sum Calculation

We add the identified prime numbers:

Sum = 61 + 67 + 71 + 73 + 79

Sum = \( 351 \)

Average Calculation

The formula for the average is:

Average = \( \frac{\text{Sum of prime numbers}}{\text{Count of prime numbers}} \)

There are 5 prime numbers in our list.

Average = \( \frac{351}{5} \)

Average = \( 70.2 \)

Final Average Result

The average of all prime numbers between 60 and 80 is calculated to be 70.2.

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