Thirteen is a good one for, like, the ACT and SAT, the PLAN test. There's a skill to turning word problems into equations. Here's what I was taught, to write out the sentence and then look at the key words. Words like is. Is is equals.
Equals is is.
You get the point. He writes a problem on the board, then disappears for a moment inside the small office adjoining the class. When he comes back out, he walks over to junior Jen Smith and hands her a note asking if anything's wrong. She's a talented artist, a gifted singer, a girl who wears her bangs so long they veil her eyes and her pants so baggy they swallow her feet. She opens the note, shakes her head and hands it back, expressionless.