Question 21

Once inside the tunnel they walk a long distance before coming to a halt in front of a huge gate with a chest placed near it. It is written that the key of the gate is in the chest.

Question 21 :
The chest is locked by a number of padlocks. All padlocks must be unlocked in order to open the chest. 12 Copies of the keys to the padlocks are distributed lie next to it, such that any group of 7 or more keys can open the chest should they choose to do so, but any group of less than 7 cannot. What is the minimal number of padlocks required to achieve this?
(a) 330 (b) 495 (c) 792 (d) 924

Answer is c)792

Ans is 924

792 must be correct

Ans is 792

Plz explain