On-Air Challenge: Every answer is the name of a world capital. You'll be given a four-letter word. The first two letters are the first two letters of the city's name, and the last two are the last two letters of the country's name. For example, if you were given "loin," the answer would be London, Great Britain.
Last Week's Challenge from the Grabarchuk family: Take 15 coins. Arrange them in an equilateral triangle with one coin at the top, two coins touching below, three coins below that, then four, then five. Remove the three coins at the corners so you're left with 12 coins. Using the centers of the 12 coins as points, how many equilateral triangles can you find by joining points with lines?
Answer: 25 equilateral triangles
Winner: Chris Anderson from Portland, Ore.
Next Week's Challenge from listener Henry Hook of Brooklyn, N.Y.: What number comes next in the following series: 2, 4, 6, 9, 11, 15, 20, 40, 60 and 90?
If you know the answer to next week's challenge, submit it here. Listeners who submit correct answers win a chance to play the on-air puzzle. Important: Include a phone number where we can reach you Thursday at 3 p.m. Eastern.