Sunday Challenge: Count the triangles.

视频简介

Sunday Challenge: Count the triangles. Answer to last week’s Sunday Challenge: Take any 8 cards and put them into one pile. Put the remaining 18 cards into a second pile. Now flip every card in the smaller pile over. That’s it. Why it works: If the original spread had 8 face-down cards total, then the pile of 8 cards contains some number of face-down cards — say X. That means the other pile has 8 − X face-down cards. When you flip all 8 cards in the first pile, those k face-down cards become face-up, and the 8 − X face-up cards become face-down — matching the other pile exactly. Clean, mechanical, and impossible to mess up. A classic piece of quiet magic.