If the population grows at the rate of $8 \%$ per year, then the time taken for the population to be doubled, is (Given $\log 2=0.6912$)

The area of the square increases at the rate of $0.5 \mathrm{~cm}^2 / \mathrm{sec}$. The rate at which its perimeter is increasing when the side of the square is 10 cm long, is

A tuning fork $A$ produces 5 beats per second with a tuning fork of frequency 480 Hz . When a little wax is stuck to a prong of fork $A$, the number of beats heard per second becomes 2 . What is the frequency of tuning fork $A$ before the wax is stuck to it ?

Two spherical rain drops reach the surface of the earth with terminal velocities having ratio $16: 9$. The ratio of their surface area is