This is not a simple problem . . .
A fish tank has a triangular cross-section.
It has a length of 80cm, width of 50cm and a depth of 40cm.
Initially it is full of water.
After 4 days, the depth of the water had dropped to 35cm.
The rate of evaporation is proportional to the surface area of the water.
Calculate how long the water in the fish tank takes to completely evaporatel.
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The volume of water is: . .
From the similar right triangles: . .
Substitute  into : .
. . Then: . .
We are told that is proportional to the surface area: .
. . The surface area is: .
. . Subtitute : .
Then: . .
Equate  and : .
. . The height function (so far) is: .
Hence, the height function is: .
When is the tank empty ?
Therefore, it takes .