PrepTest 116 · Section 3 · Question 18

The Cloudy Fish Tank

Lulu keeps a clipboard next to the classroom fish tank. Until last spring, Tommy cleaned the tank filter every single week. Since then, he has cleaned it every two weeks, because he says his arm gets tired. Lulu has a rule written in big letters at the top of her clipboard: the less often the filter gets cleaned, the cloudier the water gets, and the easier it is for green slime to grow on the glass. Marcus reads the clipboard and gasps. 'The fish are doomed! They must be sick right now!' Priya sips her juice box. 'Slow down, detective. The clipboard says nothing about sick fish. It only lets us compare. Back when Tommy cleaned every week, a normal week had clearer water than a normal week has now.' Lulu nods and underlines 'compare' twice.

Until last spring, the filter in the class fish tank was cleaned every week. Since then it has been cleaned every two weeks. The less often the filter is cleaned, the cloudier the water becomes and the easier it is for slime to grow on the glass.

Assuming everything above is accurate, what conclusion gets the best backing from it?

The Case of the Tall Book Cart

In a fictional town court, a library is accused of losing patrons' books. The witness explains: until last year, returned books were put back on shelves every day. Since then, every other day. And the less often books get shelved, the taller the pile on the return cart. The plaintiff's lawyer shouts, 'So patrons lose more books now!' Mr. Whitfield snaps his suspenders. 'Your Honor, the rule only tells us about the pile. It says nothing about lost books. What it does prove is modest: last year, on an ordinary day, the cart pile was shorter than it is today.' The judge writes that down and nothing more. 'Counsel, I will take the comparison. The rest needs evidence.'