Answer
7 minutes.
Work Step by Step
The sequence of times at which train stops at the station is
$6:00\text{ A.M},6:12\text{ A.M},6:24\text{ A.M},6:36\text{ A.M},6:48\text{ A.M},7:00\text{ A.M},7:12\text{ A.M},7:24\text{ A.M},7:36\text{ A.M},...$
The nearest time after $7:29\text{ A.M}$ in the sequence is $7:36\text{ A.M}$ and the difference between both is 7 minutes.
So, if we arrive at the station at $7:29\text{ A.M}$, we must wait 7 minutes for the train.
