What is the third proportional to 16 and 36 ?

f C is the third proportional to A and B, then A/B = B/C If x is the third proportional to 16 and 36, then 16/36 = 36/x => 4/9 = 36/x => x = 36 * 9/4 => x = 9 * 9 => x = 81