The thing is you cannot add what one person has to what someone else has paid, you have to deduct it.

Each person paid QR9:             QR9 x 3 = 27

Waiter has QR2:                              -  2

Difference:                                          25  - Bill

Then sum it up,

Each person has QR1                QR1 x 3 = 3

The waiter has QR2                                2

Bill is QR25                                           25

The total is                                        QR30