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
