Jawaban:

the probability of picking two red pens is 6/36 = 1/6.

Penjelasan dengan langkah-langkah:

The probability of an event occurring is the number of ways the event can occur, divided by the total number of possible outcomes. In this case, there are 4 pens that are red and 5 pens that are yellow, so there are a total of 9 pens.

If we pick two pens without replacement (i.e., we do not return the first pen to the box before picking the second pen), there are a total of 9 * 8 = 72 possible combinations of two pens. Of these, there are 4 * 3 = 12 combinations that consist of two red pens. Therefore, the probability of picking two red pens is 12/72 = 1/6.

Alternatively, we could also solve this problem by using the combination formula, which says that the number of ways to choose k items from a set of n items is n! / (k! * (n-k)!). In this case, the number of ways to choose 2 pens from a set of 9 pens is 9! / (2! * (9-2)!) = 36. Of these, the number of ways to choose 2 red pens from a set of 4 red pens is 4! / (2! * (4-2)!) = 6. Therefore, the probability of picking two red pens is 6/36 = 1/6.