The rate of change of the area of a circle with respect to its radius $r$ at $r = 6 \text{ cm}$ is

$A$ cylindrical tank of radius $10 \ m$ is being filled with wheat at the rate of $314 \ m^3/h$. The rate at which the depth of the wheat is increasing is: (in $m/h$)

$A$ swimming pool is to be drained for cleaning. If $L$ represents the number of litres of water in the pool $t$ seconds after the pool has been plugged off to drain and $L=200(10-t)^{2}$,how fast is the water running out at the end of $5$ seconds? What is the average rate at which the water flows out during the first $5$ seconds?

$A$ rod $AB$,$13 \text{ feet}$ long,moves with its ends $A$ and $B$ on two perpendicular lines $OX$ and $OY$ respectively. When $A$ is $5 \text{ feet}$ from $O$,it is moving away at the rate of $3 \text{ feet/sec}$. At this instant,$B$ is moving at the rate of:

The sides of an equilateral triangle are increasing at the rate of $2 \ cm/s$. Find the rate at which its area is increasing when each side is $10 \ cm$.

Consider an expanding sphere of instantaneous radius $R$ whose total mass remains constant. The expansion is such that the instantaneous density $\rho$ remains uniform throughout the volume. The rate of fractional change in density $\left(\frac{1}{\rho} \frac{d \rho}{dt}\right)$ is constant. The velocity $v$ of any point on the surface of the expanding sphere is proportional to