wrote in message
oups.com...
If the weather forcast says we have a 50% chance of rain today, and a
40% chance of rain tomorrow, what is the chance that we will have rain
during that 48 hour period? And what is the probability that we will
have rain each day during that 48 hour period?

I've tried:

P(A||B) = A + B - A * B; # Is this approach right or wrong?

P(A&&B) = (A + B) / 2;

Please, help so that I may better understand what our weather man is
really saying!


Before doing calculations we need to know what is meant by "50% chance of
rain today".
If a forecast for a region says 50% chance today it could mean that

1. It will rain over 50% of the region today
2. There is a 50% chance that any every point in the region will have rain
today
3. There is a 50% percent chance that it will rain somewhere in the region
today.

Interpretations 1 and 2 are almost (but not quite) mathematically
equivalent, but interpretation 3 is not.