100 doors revenge 75, you need to find 2 of keys, the clue is symbol blue circle from smallest to bigger size on the door. the clue is means, you need to sort locker from the smallest to the greatest by tap on the locker to find the key. check picture below for the sequence

100 doors revenge 75 walkthrough 100 Doors Revenge 75 Walkthrough

The above picture shows the sequence to find the first key, when you find the first key in the locker 7, grab the key and put in locker 1, and close it

the sequence to find the key to the locker 2, locker one does not count, so

Locker 2 becomes 1
Locker 3 becomes 2
Locker 4 becomes 3
Locker 5 becomes 4
locker 6 becomes 5
locker 7 becomes 6

after you find key again, grab it and use on the door