Probability, counting and odds
Multiply for 'and', add for 'or' when the outcomes cannot overlap, and subtract the overlap when they can.
A worked example
Drawing two red cards from a deck without replacement: (26/52) x (25/51) = 24.5%.
The mistake people actually make
Odds compare successes to failures, so odds in favour of 4 : 1 mean a probability of 4/5, not 4/1. Without replacement, the second denominator drops by one.