i was just looking over my notes and i do not understand this question entirely

a cone with height 10ft and top radius 5ft is filled with oil which weighs 57lb/ft^3. Find work done in pumping out the oil.

I know that the interval being sliced is [0,10] because the oil fills the entire tank.

I also understand why the the distance is (10-y)

what i do not understand according to my notes is where (y^2) comes from. Or where the radius comes into play?

If it is coming from the formula for a cone 1/3pi*r^2 then why does my notes say the final answer is

i thought it should be