At 6 o'clock, the hour hand is at 6 and the minute hand is at 12, so they are 30 min apart.

Now, to be together, the minute hand has to gain 30 minutes over the hour hand.

Time taken to gain 55 minutes = 60 min.

I can't understand this last sentence explianed by Mr. Santosh Sir, plz clarify sir

Let us see the clock in the following figure :

The minute hand and the hour hand are differ by 30 minutes at 6 o'clock.

Now see the another clock where both the hands (i.e. minute hand as well as hour hand) are at the same position at 12pm as shown below :

Rotate them by 60 minutes i.e. 1 hour we will get 

Now, the hour hand is at 1 and minute hand is at 12 after rotating by 60minutes.

Taking in clock wise direction, see what is the gain in 60 minutes.

Obliviously it is 55 minutes as the hour hand is at 1 and the minute hand is at 12 and the direction is clockwise.

Hence, we write that,

Time taken to gain 55 minutes = 60 minute.

Hope you will get it now !