If is an integer (a whole number), then the expression represents an even number, because even numbers are the multiples of 2. The expressions 2 n − 1 and 2 n + 1 can represent odd numbers, as an odd number is one less, or one more than an even number.
Since a product in maths means the result after multiplying numbers, you need to multiply 2 prime numbers and get 4.
If we look at the prime numbers: 2, 3, 5, 7, 11, etc, we see that the only number we can multiply to get 4 is 2, or, in other words, the only multiplication of prime numbers we can do to get four is 2*2=4.
The probability that it doesn't land on 4 would be 3/8. This is because theres a 5/8 chance it lands on 4 so just do the opposite which would be 3/8.
