The end of the tank is a circle. Place the circle on a coordinate system with the center at the origin. Then the equation of the circle is

A slice of gasoline perpendicular to the y-axis could be approximated by a right rectangular prism with dimensions 2x, 4 and dy.

F=(density of gasoline)(volume)

In the coordinate system a point that is 5 above the top of the tank would have y-coordinate 6.5

The distance the slice would have to travel is 6.5-y