Now we will take care of the task, which at first glance nothing to the sound, not to physics is not. Nevertheless, I will ask you to give her attention: it will help you easier to understand further.
You probably have already met with this problem in one of its numerous modifications. From Moscow to Vladivostok every afternoon comes the train. And every afternoon from Vladivostok to Moscow also leaves the train. The move takes, say, 10 days. The question is: how many long-distance trains will meet you during the trip from Vladivostok to Moscow?
Often meet 10. But the answer is incorrect: you will meet not only 10 trains that will come from Moscow after your departure, but also those which, at the moment of your departure was already on the way. Therefore, the correct answer is 20, not 10.
Next. Each train to Moscow carrying the latest issue of the newspaper. And if you want news, you will certainly be on the stations hard to buy Newspapers. How much fresh rooms Newspapers will buy you 10 days?
You would now correct answer: 20. Every encounter you train carries new rooms, as well as you will find 20 trains, the numbers will read too 20. But you are traveling just 10 days, then would you read a daily newspaper twice a day!
The conclusion is a bit unexpected, and you will probably not immediately believed him, if you had not happened in practice to verify its correctness. Let us recall that during the two-day crossing from Sevastopol in Leningrad you had time to read the Leningrad newspaper, not two, but four days: the two numbers that are already in Leningrad by the time of your departure, two rooms, which are published within two days.
So, you know, for whom daily Metropolitan Newspapers are published twice a day: for passengers on all trains traveling to the capital.