Let a kind of bacteria grow following the function $f(t) = t^4$,where $t$ is given in seconds. If the rate of growth of the bacteria after $t_0$ seconds is $4000 \text{ units/second}$,then $t_0 =$

The surface area of a spherical bubble increases at a rate of $2 \text{ cm}^2/\text{s}$. The rate at which the volume of the bubble increases when the radius is $6 \text{ cm}$ is . . . . . . $\text{cm}^3/\text{s}$.

$A$ circular disc of radius $3 \text{ cm}$ is being heated. Due to expansion,its radius increases at the rate of $0.05 \text{ cm/s}$. Find the rate at which its area is increasing when the radius is $3.2 \text{ cm}$.

$A$ kind of bacteria grows by $t^3$ in $t \ s$. The time taken for the rate of growth of the bacteria to become $1200 \ \text{per } s$ is: (in $s$)

If the area of a circle increases at the rate of $\frac{1}{\sqrt{\pi}}$ sq. units/sec, then the rate (in units/sec) at which the perimeter of the circle changes, when perimeter is $\sqrt{\pi}$ units, is

The length and breadth of a rectangle are $x \text{ cm}$ and $y \text{ cm}$ respectively. If the length decreases at the rate of $5 \text{ cm/min}$ and the breadth increases at the rate of $3 \text{ cm/min}$,then the rates of change of the perimeter and area respectively when $x = 5 \text{ cm}$ and $y = 2 \text{ cm}$ are: