Who Gets the Charger
iPhones are nice - but they need to be charged like everything else. Who is going to lose power first?
Besides solving the original problem, what else can you figure out about the situation?
What if the initial conditions change? What charge would the newer phone have to run out at the same time? If the old phone is at full charge, what charge on the new phone would last as long? At what starting conditions is there no crossover?
Original problem at: http://www.imakenews.com/eletra/go.cfm?z=themathmom%2C595863%2CbmP9VvPR%2C5753292%2Cblc9S0M