Shortcut Trick
The prime numbers strictly between 5 and 12 are 7 and 11.
For any set of prime numbers, the Least Common Multiple (LCM) is always their product because they have no common factors other than 1.
LCM = 7 × 11 = 77.
∴ The correct answer is 77.
Alternate Method
Given: The range of numbers is between 5 and 12.
Formula Used: LCM (a, b) = Product of numbers if they are coprime.
⇒ List the integers between 5 and 12: 6, 7, 8, 9, 10, 11.
⇒ Determine which of these are prime numbers: 7 and 11.
⇒ 6 is composite (2 × 3), 8 is composite (23), 9 is composite (32), and 10 is composite (2 × 5).
⇒ 7 and 11 are prime as they have no divisors other than 1 and themselves.
⇒ To find the LCM of 7 and 11, list their multiples:
⇒ Multiples of 7: 7, 14, 21, 28, 35, 42, 49, 56, 63, 70, 77, ...
⇒ Multiples of 11: 11, 22, 33, 44, 55, 66, 77, 88, ...
⇒ The Least Common Multiple is 77.
∴ The correct answer is 77.
Additional Information
Prime Numbers and LCM
For any set of distinct prime numbers, their HCF is always 1 and their LCM is always the product of those numbers.
The Product Rule
For any two numbers a and b: LCM(a, b) × HCF(a, b) = a × b. Since HCF of two different primes is 1, LCM = a × b.
Prime Numbers (1 to 20)
The prime numbers up to 20 are 2, 3, 5, 7, 11, 13, 17, and 19. Note that 1 is neither prime nor composite.