That seems straightforward, but I need to point out that this is the average speed. That's what you get when you divide the distance traveled (Δs) by the time it takes (Δt). Do not confuse this with the average velocity. Normally, when we say average velocity we are talking about the vector velocity. This depends on the starting and ending position of the motion. If I run to the tree and back, my average velocity would be the zero vector.

Now I just need to estimate the distance and the time and boom—I have the average Quicksilver speed. Of course that sounds simple, but there are some problems. I'm going to need to estimate some things. Such as:

The total time it takes Quicksilver to save all these people.

The number of trips he makes in and out of the X-Men Mansion.

The percent of time he is running vs. the time he spends just making silly gestures and playing with stuff.

Let's start with the easiest estimation—the time. Quicksilver seems to (somehow) notice the explosion right when it starts inside the mansion. I have no idea how he knows it exploded. He can't possibly hear it because the sound would travel with the shockwave. Also, he can't see the explosion since it's inside the mansion. Oh well. I guess it doesn't matter. But still—the total time has to be the time it takes for the shockwave from the explosion to expand to the outside yard. That's how much time Quicksilver has to get everyone out.

So, just how fast is an explosion? Of course this is no ordinary explosion. It's somehow connected to his nemesis, Apocalypse—it's not some conventional chemical-based explosive. But still, that's a good place to start. The speed of an expanding shockwave is called the detonation velocity. Luckily, Wikipedia has a table of detonation velocities.

The slowest detonation velocity is from ammonium nitrate with a speed of 2700 m/s and the highest (DDF) is around 10,000 m/s. In order to accommodate this range of speeds, I am going to use a detonation velocity of 6000 +/- 3000 meters per second. The "+/-" means "plus or minus," to display the range of uncertainty. I am going to be using this for all my estimates.

Now I need the explosion distance. Let me call this d y (for distance to the yard). I have no idea where this mansion is, so I'm just going to guess the distance is 100 +/- 50 meters. Just to be clear, with this notation that means the real yard distance is somewhere between 50 meters and 150 meters. I can put these two estimates together to get the total saving time.