Edward spends USD 21 on a bottle of vodka, a case of beer and lemonade. The vodka costs twice as much as the beer, and the lemonade costs half as much as the beer. How many dollars does the beer cost?
Each switch toggles two lights: A: 1, 2; B: 2, 4; C: 1, 3; D: 3, 4. Throw B, C, A, D to change Figure 1 into Figure 2. One switch is faulty. Which? Black=on; white=off.