Skip to main content
OlympiadHQ

Browse · MATH

Print

jmc

counting and probability intermediate

Problem

I have two 20-sided dice that each have 4 maroon sides, 7 teal sides, 8 cyan sides, and one sparkly side. If I roll both dice, what is the probability they come up the same?
Solution
This problem requires a little bit of casework. There are four ways in which the dice can both show the same thing: if they both show maroon, both show teal, both show cyan or both show sparkly. The probability of getting maroon is , so the probability that they will both show maroon is . Similarly, the probability of getting teal is , so the probability that they will both show teal is , the probability of getting cyan is , so the probability that they will both show cyan is and the probability of getting sparkly is , so the probability that they will both get sparkly is . So our answer is .
Final answer
\dfrac{13}{40}