A cube with its sides numbered 1 through 6 is rolled twice, first landing on a and then landing on b. If any roll of the cube yields an equal chance of landing on any of the numbers 1 through 6, what is the probability E that a + b is prime?
Choices: A) 0 B) 1/12 C) 5/12 D) 7/18 E) 4/9
Total # of outcomes is 6*6= 
<gadget id="calculator">6*6</gadget>
<output>36</output>
36;   Favorable outcomes:   a-b --&gt; prime   1-1 --&gt; 2;   1-2 --&gt; 3;   2-1 --&gt; 3;   1-4 --&gt; 5;   4-1 --&gt; 5;   2-3 --&gt; 5;   3-2 --&gt; 5;   1-6 --&gt; 7;   6-1 --&gt; 7;   2-5 --&gt; 7;   5-2 --&gt; 7;   3-4 --&gt; 7;   4-3 --&gt; 7;   6-5 --&gt; 11;   5-6 --&gt; 11.   Total of 15 favorable outcomes   E=15/36.   Answer: 5/12.
<result>C</result>
A cube with its sides numbered 1 through 6 is rolled twice, first landing on a and then landing on b. If any roll of the cube yields an equal chance of landing on any of the numbers 1 through 6, what is the probability E that a + b is prime?