Answer
$\frac{1}{5}$
Work Step by Step
We know that $probability=\frac{\text{number of favourable outcomes}}{\text{number of all outcomes}}$
The total number of outcomes is $6\cdot5=30$ because we can select any of the marbles as the first one and then any of the marbles that we have not selected as the first one.
The number of favourable outcomes is $3\cdot2=6$ because we can select any of the red marbles as the first one and then any of the red marbles that we have not selected as the first one.
Hence the probability here is: $\frac{6}{30}=\frac{1}{5}$
