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Umu · Feb 12, 2013, 01:47 PM

I have two dice - one is 6-sided and the other is 4-sided.
I throw the two dice. What is the probability of getting the sum 3 (I add the two results)

I figure that the probability for 1 or 2 on the 4-sided dice is 50% = 1/2
and
that the probability for 1 or 2 on the 6-sided dice is 33 1/3% = 1/3

but what do I do with that information? Do I add them: 3/6 + 2/6 = 5/6
or do I * 1/2 * 1/3 = 1/6

I don't think that any of the two answers is correct. The correct answer must be smaller - but I cannot figure out what to do

ebaines · Feb 12, 2013, 02:38 PM

There are two possble combinations that add to 3:

a) Roll a 1 on the 4-sided die and 2 on the 6-sided.
b) Roll a 2 on the 4-sided and 1 on the 6-sided.

Calculate the probability of each of these and add then tgether. Post back with what you get for an answer and we'll check it for you.

Umu · Feb 13, 2013, 04:08 AM

Probability of 1 on the 4-sided die = 1/4
Probability of 2 on the 6-sided die = 1/6

or the other way around - also 1/4 and 1/6

1/4 + 1/6 = 3/12 + 2/12 = 5/12

No that doesn't seem right either??

ebaines · Feb 13, 2013, 06:26 AM

If the probability of event 'A' occurring is 1/4, and the probability of event 'B' is 1/6, and the two events are independent, then the probability of both 'A' and 'B' occurring is 1/4 x 1/6 = 1/24. So that's the probability of rolling a 1 with the first die and a 2 with the second. Can you take it from here?

Umu · Feb 14, 2013, 12:58 AM

Probability 1: Roll a 1 with the first die and a 2 with the second is 1/4 x 1/6 = 1/24
Probability 2: Roll a 2 with the first die and a 1 with the second is 1/4 x 1/6 = 1/24

Probability of getting sum 3 = 1/24 + 1/24 = 2/24 = 1/12

That sounds right - I believe.
Thanks for the help