Two fair 8-sided dice are rolled. What is the outcome of rolling a 7 OR a sum of 7?

Mathematics
Tutor: None Selected Time limit: 1 Day

I have tried doing this so many times and the answer is not 21, which I don't understand since the probability of rolling a 7 is 15 and the probability of getting a sum of 7 is 6. "or" means add if the question said "and" you would multiply (union/intersection)

Oct 23rd, 2015

Thank you for the opportunity to help you with your question!

Outcomes should not be confused with their probabilities.

The outcomes for rolling a sum of 7 are; (1,6) (2,5) (3,4) (6,1) (5,2) and (4,3) = 6 events/outcomes

The outcomes of rolling a 7 are; (7,1) (7,2) (7,3) (7,4) (7,5) (7,6) (7,8) (7,7) (1,7) (2,7) (3,7) (4,7) (5,7) (6,7) (8,7) = 15 events/outcomes

Therefore the total outcomes is 15+6 =21

Please let me know if you need any clarification. I'm always happy to answer your questions.
Oct 23rd, 2015

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