Shortcut Trick
The Highest Common Factor (HCF) of three numbers A, B, and C is determined by HCF(HCF(A, B), C).
Given HCF(A, B) = 36 and C = 18, we calculate HCF(36, 18).
Since 18 is a direct factor of 36 (18 × 2 = 36), the HCF must be 18.
∴ The correct answer is 18.
Alternate Method
Given:
Highest Common Factor of A and B = 36
The value of number C = 18
Formula Used:
HCF(A, B, C) = HCF(HCF(A, B), C)
Calculations:
⇒ HCF(A, B, C) = HCF(36, 18)
⇒ Prime factors of 36 = 2 × 2 × 3 × 3
⇒ Prime factors of 18 = 2 × 3 × 3
⇒ Common prime factors = 2 × 3 × 3 = 18
⇒ Therefore, HCF(36, 18) = 18
∴ The correct answer is 18.
Additional Information
HCF Definition
The Highest Common Factor (HCF) or Greatest Common Divisor (GCD) is the largest positive integer that divides each of the numbers without leaving a remainder.
Associative Property of HCF
For any set of numbers, the HCF can be found by taking the HCF of any two numbers first, then taking the HCF of that result and the third number: HCF(x, y, z) = HCF(HCF(x, y), z).
Divisibility Rule for HCF
If one number is a factor of another number (i.e., it divides the other number completely), then the smaller number is the HCF of those two numbers.