Intelligent Life

Various folks have tried their hand at calculating what the chances are of intelligent life in the universe (since, we haven't been able to find any on earth), so just for the fun of it I thought I would make my own attempt. A textbook I have says that there are at least 100 billion stars in the Milky Way Galaxy, and that the Milky Way is one of countless billions of galaxies. But let's just try to calculate the probability of intelligent life in the Milky Way Galaxy, accepting the estimate of 100 billion stars, or, to put it another way, How many planets we would expect to find intelligent life on at the present time in the Milky Way Galaxy?
This, of course, requires a bunch of assumptions, but I am going to claim that I am so smart that I know all of these. Given my incredible level of brilliance, I am going to declare the following:
One half of all the stars in the Milky Way Galaxy have planets orbiting them.
The average number of planets orbiting those stars that have planets is the same as in our own solar system, which is eight (since we "lost" Pluto).
One in 10 planets in the galaxy have or at one time will develop conditions suitable for the development of life.
One half of those that develop conditions suitable for life will actually develop life.
Out of every hundred that develops life, on one planet will life evolve to the point of intelligence and this will happen only once.
The average length of time that intelligent life persists on any planet on which it evolves is 5 million years.
The earth is 5 billion years old, and the earth, solar system, and galaxy are halfway through their "lifetime" (the length of time available for all this evolution and development to take place).
The smallest amount of time it takes for intelligent life to evolve is 2 billion years. After that, for the rest of the 8 billion year span, the chances are even at any point in time that intelligent life would evolve on any given planet.
All the stars in our galaxy came into being and will end at the same time (10 billion years later). [Hey, I did not say all of these assumptions were reasonable. I am just declaring them to be true, nevertheless. If you don't like my statements of truth (or my arrogance) you can go calculate your own figures!]
So then, THE QUESTION: Given that all of these statements are absolutely true because you know that I am supremely brilliant and could never be wrong, how many planets would we expect to find in our Milky Way Galaxy that currently have intelligent life?

Answer:

125,000

Calculation: Combining the first two assumptions (err...ahh... that is... true statements) with the given estimate of the number of stars, we find that there are 400 billion planets. Multiplying this number by the probabilities given in the next three statements gives us: 400 billion x 0.1 x 0.5 x .01 x 1 = 200 million planets that will develop intelligent life. Combining the next three statements we find that the chance of intelligent life existing at any point during this scenario is 5 million / 8 billion = .000625. Multiplying this by the number of planets developing intelligent life and we get 200 million x 0.000625 = 125,000 planets. The last assumption (err...ahh... once again, that is... statement of truth) does not affect our calculation, but it is necessary to make.

Therefore, I can conclude, with total confidence, that there are 125,000 planets in the Milky Way Galaxy alone that currently contain intelligent life. May we someday join them!

Okay! Okay! If you just don't accept my absolute authority on these things, then you can calculate this using your own numbers!