Problem of mathematics (number 1) :

To go to Danemark, a French college use seven bus that is 2/3 emply. To ear money, teachers decide to stop all the bus at Strasbourg and reduct the number of bus. How many bus do the school need for all the pupils have a sitting place ?

And in this bus, three pupils drink a bottle of water (1 liter). One, alone, drink 1 liter during 1 hour. The second, alone, drink 1 liter during 3 hours. The third, alone, drink 1 liter during 6 hours. How much time do they need to drink together all the bottle ?