Let's split it up to make it a little easier.
There's 50 minutes plus another 5 minutes that needs to be added to 11:55
Now, if we add the 5 minutes to 11:55, it becomes 12:00
And we simply have to add the other 50 minutes to 12:00, making it
12:50
So, 55 minutes later than 11:55 is 12:50