The oxide of a metal contains $40 \%$ of oxygen. The valency of the metal is $2$. What is the atomic weight of the metal?

The number of significant figures in $23.023$,$0.0003$,and $2.1 \times 10^{-3}$ are respectively:

Serum differs from blood in

An auditorium has a volume of $10^5 \ m^3$ and a total absorption area of $2 \times 10^4 \ m^2$. Its average absorption coefficient is $0.2$. The reverberation time of the auditorium in seconds is:

If $x + \sqrt{x^2 + 1} = a$,then $x =$

Smoke is an example of ...