Set up a proportion for both equations:
[tex] \frac{5}{3.5} = \frac{20}{x} [/tex]
The numerator represents the amount of books Jack reads, and the denominator represents the weeks it takes for Jack to read the books.
Cross multiply both fractions.
[tex]5x = 70[/tex]
Divide both sides by 5 to get x by itself.
[tex]x = 14[/tex]
Jack will take 14 weeks to read 20 books.