Question

find the mean proportional 81 and 100.

Answer 1

Let the mean proportional be x
Then,
81:x::x:100
product of means = product of extremes
therefore, 
x*x=100*81
x^2=8100
x=90

Answer 2

if a and b are constant and question is asking about mean proportional then it forms as a:x::x: b. so 81:x::x:100 x2 =8100. x=√8100. x=90