finding height of pyramid

*Read the question for 3 times and finally understood what it means*'

We know that the $\frac{1}{9}$ part of the pyramid is above water,  since the pyramid is three-dimensional, the volume of it is equal to the third power of its sides.

Now since we know that $\frac{8}{9}$ of the volume is underwater, therefore the volume of the smaller pyramid which is above the water would be 8 times of the remaining pyramid which is underwater

The height of the smaller pyramid $=x$, with the height of the remaining pyramid $=150-x$

$x^3:(150-x)^3=1:8$

Therefore $150-x$ would be $\sqrt[3]{8}=2$ times larger than $x$:

$150-x=2x$

$3x=150$

$x=50 (meters)$

Done :D

(Time spent $\approx$ 2 hours)