A body cools according to Newton's law of cooling from $100^\circ$C to $60^\circ$C in $20$ minutes. The temperature of the surroundings being $20^\circ$C, then the total time required for the body to cool down to $30^\circ$C is

If $\bar{a} = \hat{i} + \hat{j}$ and $\bar{b} = \hat{i} - \hat{k}$, then the point of intersection of the lines $\bar{r} \times \bar{a} = \bar{b} \times \bar{a}$ and $\bar{r} \times \bar{b} = \bar{a} \times \bar{b}$ is

If $\bar{a}$ and $\bar{b}$ have the same magnitude and angle between them is $60^\circ$ and their scalar product is $\dfrac{1}{2}$, then $|\bar{a}|$ is ____

If $\bar{a} = \hat{i} + \hat{j} + \hat{k}, \bar{b} = \hat{i}, \bar{c} = c_1\hat{i} + c_2\hat{j} + c_3\hat{k}$ with $c_1 = 1$, $c_2 = 2$, then value of $c_3$ such that $\bar{a}, \bar{b}, \bar{c}$ are coplanar is ____