No, this rel. is not proportional because of the addition involved ("plus 1 lb for each guest").
A proportional rel. features mult. only.
Geometric mean is just the two numbers multiplied together under a square root sign
So you get 6
Area of a square equals side squared
(A = s²)
30 = s²
√30 = s
You can use a calculator to find out that √30 ≈ 5.48
or you can use estimation (√25 < √30 < √36) which means that √30 is between 5 and 6.