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Drei Gäste im Restaurant

Now my favirite quiz is in German (help with Isabelle, thanks).

Drei Gäste im Restaurant

Wir (ich, Isabelle, und mein Freund) waren in einem Restaurant.  Wir nahmen ein Menue. Es kostete 300 Euro. Der Kellner kassierte das Geld von uns und ging in die Kueche zurück, dann sagte der Chef: ,,Heute ist der Jahrestag des Restaurants, also kostet das Menu nur 250 Euro''. Der Kellner kam an den Tisch zurück, aber er dachte: ,,Man kann 50 Euro nicht durch drei Personen teilen''. Dann sagte er: ,,Heute ist unser Jubilaeums Tag, also bekommen Sie 30 Euro zurück''. Jeder bekam 10 Euro. Wir dankten dem Kellner und gingen nach Hause. Der Kellner fühlte sich ein bisschen schuldig, aber er entscheied es zu vergessen. Aber, er bemerkte etwas seltsam. ''Jeder bezahlt 90 Euro, also ist die Summe 270. Ich habe 20 Euro von ihnen. Die Summe ist 270 plus 20 gleich 290. Aber zuerst bekam ich 300 Euro.'' Der Kellner schrieb:

   Jeder bezahlt 90 x drei = 270
+ Ich habe                            20
------------------------------------------------------
   Summe                            290

Wo sind denn die 10 Euro?

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