Water is leaking out of a barrel. In the first hour, it lost half the number of litres it held, plus one litre. In the second hour, it lost half the remainder plus another litre. In the third hour, it lost half the remainder again, plus a further litre; and during the fourth hour, half the remainder leaked out, plus one litre more. At the end of the fourth hour, the barrel was completely empty. How many litres did it contain to start with?

Answer 1

work backwards:
2(2(2(2(0+1)+1)+1)+1) = 30 liters