Simple Math Question

[rishid]

Was doing a problem and feel the answer is wrong, just wanted to get some input.

Q: What is the probability of getting a total of 8 on a roll of 2 standard dice?
A. 5/36
B. 8/36
C. 31/36
D. 0
E. none of the above

I answer E because there are 6 possibilities (2,6) (3,5) (4,4) (6,2) (5,3) (4,4)

The correct answer was A because (4,4) is supposedly the same thing, which I think this is stupid since getting a 4 on dice 1 and getting a 4 on dice 2 they are both possibillities. Also if the answer is out of /36 that means there should be at 36 possibilities, but if you count doubles as only 1 possibilities and not 2, then the divedend should be 30.

Am I correct? Feeling stupid that I think I am right but not sure.

Thanks for your time.

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[lorelei]

The answer is right. For example, there's two ways you can get a 2 and a 6, namely A=2/B=6 and A=6/B=2. You recognized that by listing (2,6) and (6,2). However, there's only one way you can get two 4s, which is A=4/B=4. So you can't count the double twice.

 

[DieselPetrolGrl]

this isnt mcat ..god please say its not

 

[stoleyerscrubz]

this will not be on the mcat but i think you need to know a little probability to do genetics like doing tree diagrams

 

[rishid]

Well if you right then...
A=1 B=1
A=1 B=2
A=1 B=3
A=1 B=4
A=1 B=5
A=1 B=6
A=2 B=1
A=2 B=2
A=2 B=3
A=2 B=4
A=2 B=5
A=2 B=6
A=3 B=1
A=3 B=2
A=3 B=3
A=3 B=4
A=3 B=5
A=3 B=6
A=4 B=1
A=4 B=2
A=4 B=3
A=4 B=4
A=4 B=5
A=4 B=6
A=5 B=1
A=5 B=2
A=5 B=3
A=5 B=4
A=5 B=5
A=5 B=6
A=6 B=1
A=6 B=2
A=6 B=3
A=6 B=4
A=6 B=5
A=6 B=6


When A=B that this shouldn't be included as 2 possibilities the the divisor should be /30 not /36...

Sorry for dwelling on this...

Thanks

 

[stoleyerscrubz]

you can only roll a "Hard" 8 only one way so answer is 5/36
12 - 1/36
11 - 2/36
10 - 3/36
9 - 4/36
8 - 5/36
7 - 6/36
6 - 5/36
5 - 4/36
4 - 3/36
3 - 2/36
2 - 1/36

sum of all probabilities = 36/36

 

[FutureDocDO]

Just pay attention to one dice. There are six surfaces. The possibility of getting any one side is 1/6, but one (1) doesn't count because you can't get a total of eight (8) with another dice. So there are five sides left. So for example if you roll a 2 and a 6 that would be 8. Chances of getting a 2 is 1/6 and chances of getting a 6 is 1/6. Take 1/6 times 1/6 equals 1/36. There are five combinations that one can roll to get a total of 8 so 1/36+1/36+1/36+1/36+1/36 = 5/36.

 

[culov]

you are right

 

[rcd]

die a and die b

a
1 bad
2 ok
3 ok
4 ok
5 ok
6 ok

chances of die a being a good number = 5/6

chances of die b being one of the required complementary rolls = 1/6

5/6 * 1/6 = 5/36

note: this probably would have been harder if you hadn't given the answer.