SAT Question 46b2e169: Probability And Conditional Probability answer and explanation with Desmos solution

It's given that a box contains $13$ red pens and $37$ blue pens. If one of these pens is selected at random, the probability of selecting a red pen is the number of red pens in the box divided by the number of red and blue pens in the box. The number of red and blue pens in the box is $13+37$, or $50$. Since there are $13$ red pens in the box, it follows that the probability of selecting a red pen is $\frac{13}{50}$. Note that 13/50, .26, .260, and 0.26 are examples of ways to enter a correct answer.