Write 50 tricky maths word problems with worked solutions

We need 50 original word problems that today's models get wrong at least some of the time. Secondary-school to first-year-university level: rates, probability, geometry, a little number theory.

Each problem needs one correct answer, a full worked solution, and a note on the trap (the step a model tends to skip). Nothing copied from textbooks or the web; we check.

We will run each problem through two models before accepting it, and pay for every problem at least one of them fails.