Shortcut Trick
List odd numbers between 70 and 80: 71, 73, 75, 77, 79.
75 is divisible by 5 and 77 is divisible by 7 (7 × 11).
The remaining prime numbers are 71, 73, and 79.
Total count = 3.
∴ The correct answer is 3.
Alternate Method
Given: Range of numbers between 70 and 80.
Formula: A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself.
⇒ Numbers between 70 and 80 are: 71, 72, 73, 74, 75, 76, 77, 78, 79.
⇒ 71: No divisors other than 1 and 71. (Prime)
⇒ 72: Divisible by 2, 3, 4, 6, 8, 9, 12, 18, 24, 36. (Composite)
⇒ 73: No divisors other than 1 and 73. (Prime)
⇒ 74: Divisible by 2 and 37. (Composite)
⇒ 75: Divisible by 3 and 5. (Composite)
⇒ 76: Divisible by 2, 4, and 19. (Composite)
⇒ 77: Divisible by 7 and 11. (Composite)
⇒ 78: Divisible by 2, 3, 6, 13, 26, 39. (Composite)
⇒ 79: No divisors other than 1 and 79. (Prime)
⇒ Total prime numbers found: 71, 73, 79.
∴ The correct answer is 3.
Additional Information
Prime Numbers (1-100)
There are exactly 25 prime numbers between 1 and 100. These are 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, and 97.
Smallest Prime Number
The smallest prime number is 2, which is also the only even prime number. All other prime numbers are odd.
Twin Primes
Twin primes are pairs of primes that differ by 2. In the range 70-80, 71 and 73 form a twin prime pair.