probablity hw help

[gnuel3]

I can't seem to get this problem.

Two 6 sided dice are rolled. What is the probablity that the total on the 2 dice is: a) a multiple of 3, b) prime

I got 7/18 for a and 16/36 for b , but those are wrong I think..

 

[EmperorIQ]

first you gotta think about the permutations
if dice 1 is a 1 then what numbers can dice 2 be in order for it to be a multiple of 3?

1: 2, 5
2: 1, 4
3: 3
so we have 5 possible pairs, and you just divide that by how many possible pairs there can be, which is 36, so 5/36??? not sure, haha

 

[Ness]

http://www.renegadeonline.net/forumcrap/prob.jpg


The orange boxes are prime numbers. The blue numbers are multiples of 3. If you can't count out the probability here, then you probably shouldn't be in a math class with probability.

Whenever you have a problem like this, the best thing to do is to draw out a table of all combinations, if the number of combinations you have is a fair size.