In solving problems like this, the most straightforward approach is to make a table. Columns are used to show the six outcomes from one of the dice, rows the outcomes from the other die. In each cell of the table put the sum of the two dice. Now go throught the table row by row, counting the values that are prime numbers. There are 36 cells in the table, and I counted 15 primes, making a probability of 15/36 = 5/9.
But my counting has been wrong before! You had better check it.
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First you need to work out the probability of rolling a prime number. The prime numbers on a die are 2, 3 and 5. Thus the probability of rolling a prime number is 3/6 which can be simplified to 1/2. The probability of rolling a number greater than 1 is 5/6. The probability of rolling one on one dice and one on the other is therefore 1/2 x 5/6 = 5/12. There are two possible ways round these options could come though. You might get the number greater than one on the first roll, and the prime on the second. Thus we need to multiply the probability by 2, which gives us the final answer of 5/6.
2:3
If the numbers are 1 to 6, there are three prime numbers in that range, a probability of 50%.
The probability is 6/36 or 1/6