In a group of 200 people, everybody has a non burning candle. On person has a match at lights at some moment his candle. With this candle he walks to somebody else and lights a new candle. Then everybody with a burning candle will look for somebody without a burning candle, and if found they will light it. This will continue until all candles are lit. Suppose that from the moment a candle is lit it takes exactly 30 seconds to find a person with a non burning candle and light that candle. From the moment the first candle is lit, how long does it take before all candles are lit?

You put two 1 dollar coins on a table, such that they touch each other (see the figure). While you fix the lower coin with your finger, you will turn the upper around it (without slipping). Somewhat later the moving coin will be back in his original position. How many rotations on his midpoint did the moving coin make?

Correct the equation above by inserting one straight line.