Question

common multiple number for 18 and 54.its answer is 9.how

Answer 1

This is because both 18 and 54 are divisible by 9.
$18 \div 9 = 2 \\ 54 \div 9 = 6$

Answer 2

as when we take LCM of 18 and 54 its pairs come 2*3*3*3 as 3 is coming 2 times as pair . and when we will get 3 ×3=9