Find the missing term of each pair of equivalent ratio. 81:18=36:?

I like to use cross multiplication for this problem

if there is 81 to 18, then you can express this as

81/18 and 36 to? will be expressed as 36/?

81/18=36/?

you multiply the numerators with the other denominator

81*?=36*18

81*?=648

if you divide both sides by 81, you'll find you answer

?=648/81 = 8

8 is your missing term