Probability and overlapping groups
Multiply probabilities for two events happening together, and subtract the overlap once when adding two groups.
A worked example
Of 60 people, 35 own a bike and 20 own a scooter, with 8 owning both. At least one is 35 + 20 − 8 = 47, so 13 own neither.
The mistake people actually make
Adding the two group percentages counts anyone in both of them twice, so subtract the overlap before taking the total away from 100%.