Calculate the depth of water in the trough when it is exactly half full I am in my last year of high school and am currently studying for my finals by going over exercises in my coursebook. I came across this exercise and have been stuck on it for some weeks now. I have asked my class mates as well as my math tutor who also wasn't able to figure it out. The question seems to defy logic and it doesn't really make much sense to me. I am beyond figuring it out for any study purposes - I am simply curious as to how on earth this question is supposed to be answered!! 
According to my text book, the answer is supposed to be 22.93 m^3.
http://i.imgur.com/6qCThGj.jpg

The equation is $$70*d+\color{blue}{\frac{1}{2}*d^2*\frac{30}{50}}=70*(50-d)+\frac{1}{2}*(50*30-d^2*\frac{30}{50})$$
The blue part: You can draw a orthogonal line from the  end of the bottom upward. You get a triangle on the right side. This (big) triangle is divided by the horizontal line in two areas. 
One of these areas is a small triangle. The height of the small triangle is d. Then you have a base (y). This base can be expressed in terms of d by using the intercept theory
$\frac{50}{d}=\frac{30}{y} \Rightarrow y=\frac{30}{50}d$
Thus the area of the small triangle is $\frac{1}{2}\cdot d \cdot \frac{30}{50}d$
The remaining area is the area of the (big) triangle minus the area of the small triangle: $\frac{1}{2}*(50*30-d^2*\frac{30}{50})$
Like I commented, your solution is wrong.
