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This problem trains for: SAT-I, AMC-8, GMAT, AMC-10.

Two fair dice are rolled one time. If the probability to roll the sum that came up on of the top faces is 5/36 then what is the probability for one of the top faces to show a 3?

There are only two sums that have a 5/36 probability of happening: 6 and 8. We can clearly see this by taking a look at the sample space for this experiment:

123456
1234567
2345678
3456789
45678910
567891011
6789101112

The possible sums can be formed by 10 possible pairs of numbers:

equation

Out of these pairs, only 3 contain the number 3. So there is a probability of:

equation

for the number 3 to be on one of the upturned faces.