Consider the height and radius of the sand-cone formed at time t second be y cm and x cm respectively.As per the given statement, y=61​x ⇒x=6y Volume of cone (V)=31​πx2y ​=31​π(6y)2y=12πy3​ ⇒dydV​=36πy2 Now, As, dtdV​=12 ​⇒dyd V​×dtdy​=12⇒36πy2×dtdy​=12⇒dtdy​=3πy21​⇒dtdy​=3π421​=48π1​cm/sec​