The best way for him to check his answer would be to simply plug in the value found for x into the equation.
4(2) - 6 = 2
8 - 6 = 2
2 = 2
The probability of getting a 5 from the first rolling is 1/6. The probability of getting an even number is 1/2 (= 1/6 (for a 2) + 1/6 (for a 4) + 1/6 (for a 6)). As the two rollings are independent, you can just multiply two probability values to come up with a final answer. That is, (1/6)·(1/2) = 1/12.
Note that I did not write this. This answer comes from Wyzant. I am only linking this to you so that you'll get the answer quickly.
It could sometimes be proportional. The reasoning here is that if all the items have different prices it won't be, however if the items you are ordering all have the same price they will be!
Well, volume is: length x width x height
so, 5 x 14 x 8= 560 inches
