The general solution of the differential equation $2 d x+d y=(6 x y+4 x-3 y) d x$ is

For any fixed distance, the electromagnetic force between two protons is $10^n$ times of the gravitational force between them. Then, $n=$

If $A, B$ and $C$ are three different physical quantities with different dimensional formulae, then the combination which can never give a proper physical quantity is

The driver of a bus moving with a velocity of $72 \mathrm{~km} / \mathrm{h}$ observes a boy walking across the road at a distance of 50 m in front of the bus and decelerates the bus at $5 \mathrm{~ms}^{-2}$ by applying brakes and is just able to avoid an accident. The reaction time of the driver is