An image formed by a concave mirror of focal length $6 \ cm$ is $3$ times the size of the object. What is the distance of the object from the mirror in $cm$?

If the mass defect of a nuclide is $3.32 \times 10^{-26} \ g$,its binding energy is (in $MeV$)?

Which of the following is the most important cause for the extinction of animals and plants?

$3.011 \times 10^{22}$ atoms of an element weigh $1.15 \ g$. The atomic mass of the element is

Assertion $(A)$: $NaCl$ is less soluble in heavy water than in ordinary water.
Reason $(R)$: Dielectric constant of ordinary water is more than that of heavy water.
The correct answer is

Three taps $A, B, C$ fill up a tank independently in $10 \, h$,$20 \, h$,and $30 \, h$,respectively. Initially,the tank is empty. Exactly one pair of taps is open during each hour,and every pair of taps is open for at least one hour. What is the minimum number of hours required to fill the tank?