correct answer is: d) 20 metres

Explanation

Let, the whole length of the post ‘x’ metres.
According to the question,
\large\frac{1}{4} of the post length in mud.
\therefore In mud, the post length =\left(x\times\large\frac{1}{4}\right) metres.
\rightarrow \large\frac{x}{4} metres.
And, \large\frac{1}{5} of the post length in water.
\therefore In water, the post length =\left(x\times\large\frac{1}{5}\right) metres.
\rightarrow \large\frac{x}{5} metres.
\therefore In mud and water, the total post length =\large\left(\frac{x}{4}+\frac{x}{5}\right) metres.
\rightarrow \large\left(\frac{5x+4x}{20}\right) metres.
\rightarrow \large\frac{9x}{20} metres.
\therefore Above water, the post length =\left(x-\large\frac{9x}{20}\right) metres.
\rightarrow \large\left(\frac{20x-9x}{20}\right) metres.
\rightarrow \large\frac{11x}{20} metres.
Now,
\large\frac{11x}{20}\normalsize=11 metres
\rightarrow x=\left(11\times\large\frac{20}{11}\right) metres.
\rightarrow x=20 metres.
\therefore Length of the whole post 20 metres.
Ans: The whole length of the post 20 metres.

Other ways to ask this same math question are:

1. Find the total height of a post when parts of it are submerged in mud and water, with 1/4 in mud, 1/5 in water, and 11 meters above water.
2. Calculate the total length of a post given that 1/4 is buried in mud, 1/5 is submerged in water, and the top extends 11 meters above the water surface.
3. Determine the total length of a post when 1/4 of it is submerged in mud, 1/5 is submerged in water, and the top extends 11 meters above the waterline.