A water tank has the shape of a cone. the tank is 10 m high and has a radius of 3m at the top. of the water is 4 m deep, what is the surface area of the top of the water

The water is 4/10 of the full depth, so its surface has a radius that is 4/10 of the radius at the top. Of course, the surface of the water has the shape of a circle.

The area of a circle is given by

... A = π·r²

... A = π· (0.4·3 m) ² = 1.44π m²

The surface area of the top of the water is 1.44π m² ≈ 4.524 m².