When $NaCl$ is dissolved in water,the sodium ion becomes

Which of the following is the correct order of solubility of alkaline earth metal sulfates in water?

The half-life of a radioactive substance is $20 \text{ minutes}$. The time interval between $20\%$ and $80\%$ decay is ........ $\text{minutes}$.

If $10\%$ of a radioactive material decays in $5$ days,then the amount of original material left after $20$ days is approximately ...............$\%$

The group of chemicals used as pesticides is

If ${}^n P_r = 30240$ and ${}^n C_r = 252$,then the ordered pair $(n, r)$ is equal to