0:00:00.000,0:00:02.000
And here is my answer.
0:00:02.000,0:00:06.000
There are a number of ways to do this, but a simple one is to take the old mean,
0:00:06.000,0:00:09.000
multiply it by the number of observations, which gets us the sum,
0:00:09.000,0:00:13.000
add to it the new value, which gets us the new sum,
0:00:13.000,0:00:15.000
and then divide it by the new number of observations,
0:00:15.000,0:00:18.000
which is just one more than the old one.