Consider AB be the ladder and length of ladder is 5 cm . So AB=5 cm . Let C is the junction of wall and ground, let CA=x meters, CB=y meters So, according to the equation: As x increases, y decreases  and dtdx​=2 cm/s In △ABC AC2+BC2=AB2 [Using Pythagoras theorem] x2+y2=25 ​2xdtdx​+2ydtdy​=02x(2)+2ydtdy​=0​ ​2ydtdy​=−4xatdy​=y−2x​….​ When x=4,16+y2=25 y=3 [From equation(1)]From equation (2), dtdy​=3−2×4​=3−8​ cm/s