Question

Robert wants to arrange the books for statistics, calculus, geometry, algebra, and trigonometry on a shelf. In how many arrangements can he keep them on the shelf such that the algebra and trigonometry books are not together?

Answer 1

Answer: 72 arrangements

Explanation:

The books are:

Statistics, calculus, geometry, algebra, and trigonometry.

So we have 5 books.

We want that algebra and trigonometry are not together.

Suppose that we have 5 positions:

Now, we can start with algebra in the first position.

Now, we have 3 positions for trigonometry (3rd, 4th and 5th).

Now, once those two books are in position, we have 3 other positions and 3 other books, so for the first selection we have 3 options, for the second position we have 2 options, and for the last option we have 1 option.

The number of combinations is equal to the number of options in each selection:

3*(3*2*1) = 18

Now, if Algebra is in the second place, then for trigonometry we have only 2 possible options (4th and 5th)

and for the other 3 books again we have 3*2*1 combinations:

the total number of combinations is:

2*(3*2*1) = 12

If algebra is in the 3rd position, trigonometry has 2 options (1st and 5th)

For the other 3 books, we have 3*2*1 combinations.

The total number of combinations is:

(3*2*1)*2 = 12

in the fourth position is the same as the second position, so here we have again 12 combinations.

For the fifth position is the same as for the first position, so we have 18 combinations.

The total number of combinations is:

C = 18 + 12 +12 +12 +18 = 72