Thank you very much for your help, that's what i found too after a while.
I have another tricky one which is bothering me a lot :
Three guys are traveling in china and stop by a hotel. The hotel director announces the rooms' prices. A room for 3 is 30 RMB. They accept and each pays 10rmb. But the director suddenly realizes that he made a mistake. The room for 3 people was only 25rmb. Honest but not too much, he decides to give 1rmb back to each of the 3 guys and to keep the 2rmb resting for himself. So he gives back 3rmb et keeps 2. as a total the 3 guys will have paid only 9rmb each for the rent of the room.
The tricky thing : 3x9=27rmb. and 25rmb+3rmb=28rmb
where is the missing RMB?where did it disappeared?how is it possible
If you have an answer, i ll be really grateful!