Frogs

Here you see a pool with six happy frogs. You must exchange the positions of the brown and green frogs, using the following rules:
 a frog can only jump to an empty water-lily;
 a frog can only jump over at most one other frog, that has another color;
 a frog can only jump forward (so the green frogs jump right and the brown frogs jump left).

How can this be done?

 

Answer:

Number the water-lilies from left to right from 1 up to 7. Then jump as follows:
 the green frog on place 3,
 the brown frog on place 5,
 the brown frog on place 6,
 the green frog on place 4,
 the green frog on place 2,
 the green frog on place 1,
 the brown frog on place 3,
 the brown frog on place 5,
 the brown frog on place 7,
 the green frog on place 6,
 the green frog on place 4,
 the green frog on place 2,
 the brown frog on place 3,
 the brown frog on place 5,
 the green frog on place 4.
Note: it is also possible to mirror the above solution, starting with the brown frog on place 5.