If the radius of a nucleus with mass number $125$ is $1.5 \text{ fermi}$,then the radius of a nucleus with mass number $64$ is: (in $\text{ fermi}$)

The density of a mixture of $He$ and $O_2$ gases at $27 \, ^oC$ and $760 \, torr$ is found to be $0.543 \, g \, dm^{-3}$. What is the mass percentage of helium in the mixture?

$A$ horizontal force of $10\, N$ is necessary to just hold a block stationary against a wall. The coefficient of friction between the block and the wall is $0.2$. The weight of the block is ........ $N$.

The line among the following which touches the parabola $y^2=4ax$ is

Which of the following statements are true about acceleration due to gravity $(g)$ of the earth (where $r$ is distance from the centre of the earth and $R$ is the radius of the earth)?
$(a)$ $g$ decreases in moving away from the centre,if $r > R$.
$(b)$ $g$ decreases in moving away from the centre,if $r < R$.
$(c)$ $g$ is zero at the centre of the earth.
$(d)$ $g$ at the equator decreases,if the earth stops rotating about its axis.

If $P(1, 2)$,$Q(4, 6)$,$R(5, 7)$,and $S(a, b)$ are the vertices of a parallelogram $PQRS$,then: