I was going to post this under the "Maths in Aviation" subject a few days back, but since that really referred to flying itself, I felt this required a topic by itself.
If you want to predict what growth rates will do to certain airlines, airports or even city pairs, there's a formula you can use to do this. Say, for example, you want to work out how many pax. Aer Lingus (which currently carries 6m pax pa) will carry in 10 years time at a specific rate, say 5% for simplicity, you could use this formula:
(1.05) to the power of ten = 1.62889.
1.62889 x 6m = 9.77m pax
You need a scientific calculator to do this. There's a key marked x^y on the scientific calculator under the accessories section of the "Programs" section of Microsoft.
A good test of this is to go to the Airbus or Boeing sites and download their Global Market Forecasts. This gives growth rates for the next 10 and 20 years. Have a go; it's actually quite fun, once you get the hang of it. And I didn't even do Honours Maths for the Leaving Cert.!