Can you give answers for the list of questions asked in interview?

1.There is one primary db server in india and standby db server in Denmark.

There is a gap in archive log generation.Can you tell me what could be the reason for this gap .

2.Yesterday one user ran co-related sub query which got completed in 20 seconds.The same query when he tried today it took 1 minutes to retrieve the result.

How to find out the root cause of this above issue ?

3.A datafile is missing somehow.Is it possible to take a backup of missing datafile ?