Given marbles are:

$10\to R, 30\to W, 20\to B, 15\to O$

We know that, $P\left(E\right)=\frac{\text{Number of favourable outcomes}}{\text{Total number of outcomes}}$

Also, $P\left(A \text{and} B\right)=P\left(A\right)\times P\left(B\right)$, where $A \text{and} B$ are independent events.

Probability of drawing first marble red and then white is given by,

$P\left(E\right)=\frac{10}{75}\times \frac{30}{75}$

$\Rightarrow P\left(E\right)=\frac{4}{75}$