Answer:
The other answer I saw is the same way that I used to explain in my class as well. But, there's an alternative way to reach the answer as well
First thing that we should know is that if there are 'n' units kept in a row, then there are 'n+1' gaps generated by them. So, what we should understand is that when there is no item there is 1 gap, when there is one item there are 2 gaps, and so on.
Let us use that concept here, with a small variation: the 5 shelves are 5 initial gaps. The first book has 5 choices (5 initial gaps) to go to. Once we have placed this book, there are now 6 gaps on which the 2nd book can be placed. After the 2nd book is placed, there are 7 gaps in which 3rd book can be placed, and so on.
So, the total arrangements will be = 5*6*7*...24 [as there are 20 books] = 24P20
Rewriting the equation as a quadratic equation equal to zero:
x^2 - x - 30 = 0
We need two numbers whose sum is -1 and whose product is -30. In this case, it would have to be 5 and -6. Therefore we can also write our equation in the factored form
(x + 5)(x - 6) = 0
Now we have a product of two expressions that is equal to zero, which means any x value that makes either (x + 5) or (x - 6) zero will make their product zero.
x + 5 = 0 => x = -5
x - 6 = 0 => x = 6
Therefore, our solutions are x = -5 and x = 6.
18+x=74
Meaning 18 plus Diego’s age is 74. If we subtract 18 from 74 we will find Diego’s age.
