A $25m$ long ladder is placed against a vertical wall such that the foot of the ladder is $7m$ from the feet of the wall. If the top of the ladder slides down by $4cm$, by how much distance will the foot of the ladder slide ?

Before sliding, the situation is represented by a right triangle with hypotenuse $25$, and horizontal arm $7$, so that the vertical arm is $24$. After sliding, the situation is represented by a right triangle with hypotenuse $25$, and vertical arm $20$, so that the horizontal arm is $15.15-7=8$ so that (e) is the correct choice.