The number of four letter words that can be formed using letters of the word BARRACK

A stone is dropped into a quiet lake and waves move in circles at speed of $8 \mathrm{~cm} / \mathrm{sec}$. At the instant when the radius of the circular wave is 12 cm . how fast is the enclosed area increasing?

The area (in sq. units) of the region bounded by the curve $x^2=4 y$ and the straight line $x=4 y-2$ is

If the half life of substance is 5 years, then the total amount of the substance left after 15 years, when initial amount is 64 gms is