Question

g Four distinct numbers are to be selected from the list {−5, −4, −3, −2, −1, 1, 2, 3, 4}. In how many ways can this selection be done so that the product of the four numbers is positive?

Answer 1

Answer: 66 possible combinations.

Explanation:

To have a positive product we have 3 situations.

The 4 numbers are positive:

if the "order" of the selection does not matter, then we have only one solution here:

1, 2, 3 and 4.

Second case, we have two negative numbers and two positive numbers.

Here we can use the fact that in a group of N objects, the number of different combinations of K objects (where K ≤ N) is:

`C = (N!)/((N -K)!*K!)`

Here we have 5 negative numbers and we want to make groups of 2, so the possible combinations are:

`C = (5!)/(3!*2!) = (5*4)/(2*1) = 2*5 = 10`

And we have exactly the same for the other two positive numbers, but in this case we have N = 4 and K = 2.

`C = (4!)/(2!*2!) = 6`

The total number of combinations is the product of those two:

C = 10*6 = 60 combinations

Now, the last option is that the 4 numbers are negative numbers, so here we have 5 negative numbers and we want to make groups of 4.

`C = (5!)/(1!*4!) = 5`

So in total, we have: 1 + 60 + 6 = 66 possible combinations.