Bunuel wrote:

A collection of books went on sale, 4/5 of which were sold at $6.40 each. If none of the remaining 25 books were sold, what was the total received for the books that were sold?

A. $100

B. $160

C. $320

D. $480

E. $640

Total received =

(# of books sold) * (price per book)

1) Number of books

total?

Fraction of all books not sold: \(\frac{1}{5}\)

Number of all books not sold = 25

\(\frac{1}{5}\) of all books = \(25\)

All books = \(25 *\frac{5}{1} = 125\)

2) Total books

sold?*

(All books) - (# not sold) = (# sold)

(125 - 25) = 100 books sold

3) Total

money received?

100 books * $6.40 per book =

$640Answer E*

You can combine steps to find total books sold.

\(\frac{PartUnsold}{#Unsold} = \frac{PartSold}{#Sold}\)

\(\frac{\frac{1}{5}}{25} = \frac{\frac{4}{5}}{?}\)

Numerator is multiplied by 4. Do the same to the denominator: 25 * 4 = 100 sold. Or cross multiply
_________________

In the depths of winter, I finally learned

that within me there lay an invincible summer.

-- Albert Camus, "Return to Tipasa"