If you look at the attached graphic, you will see that the altitude (or height) of an equilateral triangle is:
Height = side length * sq root (3) / 2
Area = side^2 * sq root (3) / 4
0.133 because 5 mean you have to round up so it is 0.133
Let's create a variable to represent the size of Ruben's book collection prior to his buying the new books.
B = number of books in Ruben's collection before buying new ones
So, we know that when he bought 6 books, this increased his collection by 12%.
12% of his book collection would be represented by 0.12B. Therefore, we know that:
0.12B = 6
Divide both sides by 0.12, to get to B = 50.
However, you are being asked how many books does he have now. B represents the number of books he had before. Therefore you want to add back the 6 books he just bought.
The answer is 50 + 6 = 56
Since one pound is 16 ounces, it would cost .3*16, or $4.80 to buy one pound of peanuts. $5 is enough and there would be $0.20 left over.
