Answer = π L r = π * 13 * 5

note that L is the not the vertical height 12, it is the slanted height instead, which is 13. (by Pythagorean theorem 13^2 = 12^2 + 5^)  

The formula for volume is one most kids should memorise by heart.

pi r^2 h, which should be 3.142 x 25 x 12

Then use the formula for cone 1/3 pi r^2 h, which should be 1/3 x 3.142 x 100 x h