• A smartphone and a case cost $110 in total. The phone costs $100 more than the case. How much does the case cost?
  • It takes 8 printers 8 minutes to print 8 brochures. How long would it take 24 printers to print 24 brochures?
  • On a field there is a patch of weeds. Every day the patch doubles in size. It takes 30 days for the patch to cover the whole field. How long did it take for the patch to cover half the field? The economist Shane Frederick gave these questions (with different examples) to thousands of university students. Five out of six got at least one of them wrong; one in three got them all wrong.


  • $5
  • 8 minutes
  • 29 days

— Steven Pinker, Rationality