Magnifying power of a simple microscope is (when final image is formed at $D = 25 \ cm$ from eye)

Which of the following substances is known as $Oil$ $of$ $Wintergreen$?

If the vectors $a \hat{i}+\hat{j}+\hat{k}$,$\hat{i}+b \hat{j}+\hat{k}$,and $\hat{i}+\hat{j}+c \hat{k}$ $(a \neq 1, b \neq 1, c \neq 1, a \neq b \neq c)$ are coplanar,then the value of $\frac{1}{1-a}+\frac{1}{1-b}+\frac{1}{1-c}$ is:

If the slope of one line is twice the slope of the other in the pair of straight lines $ax^2 + 2hxy + by^2 = 0$,then $8h^2$ is equal to (in $ab$)

Which substance is used as a smoke screen in warfare?

The same retarding force is applied to stop a train. The train stops after $80 \ m$. If the speed is doubled,then the distance will be