What is the percentage of lead in a lead pencil (in $,\%$)?

Amongst $LiCl, RbCl, BeCl_2$ and $MgCl_2$,the compounds with the greatest and least ionic character respectively are:

What is the study of living organisms called?

The following diagram indicates the energy levels of a certain atom. When the system moves from the $4E$ level to the $E$ level, a photon of wavelength $\lambda_1$ is emitted. The wavelength of the photon produced during its transition from the $\frac{7}{3}E$ level to the $E$ level is $\lambda_2$. The ratio $\frac{\lambda_1}{\lambda_2}$ will be:

The image of the line $x+y-2=0$ in the $y$-axis is

$100 \ g$ of water is heated from $30^\circ C$ to $50^\circ C$. Ignoring the slight expansion of the water,the change in its internal energy is ...... $kJ$ (specific heat of water is $4184 \ J/kg/K$).