When zeolite (hydrated sodium aluminium silicate) is treated with hard water,the sodium ions are exchanged with:

In a series of $2n$ observations,half of the observations are equal to $a$ and the remaining are equal to $-a$. If the standard deviation of the observations is $2$,then what is the value of $|a|$?

$A$ particle executes a simple harmonic motion of time period $T$. Find the time taken by the particle to go directly from its mean position to half the amplitude.

What does orthoboric acid contain?

$A$ body cools from $62^{\circ}C$ to $50^{\circ}C$ in $10$ minutes and to $42^{\circ}C$ in the next $10$ minutes. The temperature of the surrounding is ........... $^{\circ}C$.

$S_1, S_2, \ldots, S_{10}$ are the speakers in a conference. If $S_1$ addresses only after $S_2$,then the number of ways the speakers address is