How many overs? Hitting an animal.

[sunlover14]

Fairly new to umpiring, so just wanted to clarify a couple of things.

In a 50 over game, there's 5 hours 10 mins of play available after deductions.

* The rule is that each over = 4 mins.

* So, 5 hours is 75 overs, but what about 10min, because that's 2.3 overs? Is it round up to nearest number, which is 3? 

* If so, total overs to be bowled = 78 (39 overs per team)

* What would the 3 powerplay overs be in a 39 over innings? 8, 23, 8?

Finally, if the ball is heading to the boundary but hit's a stray animal and goes for 4, is it still a boundary? Or is it a dead ball?

[sillypoint]

Some comments on the matters you raise:
1.  In most competitions there are competition rules to specify these matters, and you need to understand and follow those rules. The Laws themselves do not cover the specifics of starting times, innings length, etc, only how play is to be conducted within the parameters specified by the local rules.
2.  You would almost always be required to round part overs up to whole overs, so 10 minutes would generally translate into 3 overs if the match times are as you describe. But if the local rules implied, say, 2.5 minutes per over then 10 minutes would represent 4 overs, etc.
3.  The additional 10 minutes may, depending on the local rules, be there to provide for the required 10 minute interval between innings.
4.  There is no such thing, in the Laws of Cricket, as a power play. These matters should be covered in the local competition rules.
5.  Anything not covered by the competition rules is determined according to the Laws of Cricket.
6.  Contact with a stray animal on the field would be considered to be an accidental obstruction. In the scenario you describe the ball crosses the boundary anyway, so the obstruction would be irrelevant. The more interesting scenarios to consider would be (1) if the intervention of the animal obstructed a fielder's ability to stop the ball before it reaches the boundary; or (2) it deflected the ball in such a way as to prevent it running to the boundary. In these situations it would be the responsibility of the umpires to determine the outcome—unless, of course, there is some specific provision in the local rules. The umpires would have to determine whether the intervention of the animal was inherently unfair or whether it was simply unfortunate. The main thing to consider would be how likely a different outcome was. For example, if it was obvious that the ball was running fast enough to reach the boundary and that no field had a realistic opportunity to have stopped it then you might decide to award the boundary anyhow.