Answer
$\frac{1}{4}$
Work Step by Step
To find the probability, first add up all the marbles in the box.
$1$ $+$ $1$ $+$ $2$ = $4$
Then see how many red marbles are in the box: $1$.
So the probability would be the number of red marbles in the box over the number of total marbles in the box: $\frac{1}{4}$