St Petersburg paradoxt. A fair coin is tossed repeatedly. Let T be the number of tosses until the first head. You are offered the following prospect, which you may accept on payment of a fee. If T = k, say, then you will receive £2*. What would be a 'fair' fee to ask of you?

Pet Owners According to the Humane Society, in 2015, 65% of U.S. households owned at least one pet, and 42% of households who did own pets owned at least two. The U.S. Census Bureau tells us that there were 117 million households in 2015. How many households owned at least two pets? Report your answer in millions rounded to two places.

In Example 5, after how many days are 250 students infected?