Answer: Option a) 3
Step-by-step explanation:
The formula for calculating combinations is as follows

Where "n" is the amount of items in a set and you can choose "r" from them
3C1 reads as: The combination of 1 in 3. You have a set of 3 elements and choose 1 of them.

So :

I hope this helps you
Area=length.width
Area=5.10
Area=50
Let b be the number of books in a box. Then we can say that the total number of books is equal to the number of boxes times the number of books per box.
7b=126 divide both sides by 7
b=18
So there are 18 books per box.