Question

The highest common factor of two DIFFERENT numbers 21 and Δ is 21.

Answer 1

Step-by-step explanation:

it is a multiple of 21.

the highest common factor of 2 numbers is the longest combination of prime factors they have in common.

the prime factors of 21 are simply

3×7

if another number A shares this as the longest combination of prime factors, that means it contains 3×7 itself in its prime factors.

and that means this number A can only be a multiple of 3×7 (= 21), as other prime factors present would be the factor of 21.