I'm stuck on this question
Suppose that 40% of visitors to a city visit the mountains, 70% visit the parks and 20% visit both sites. One visitor is randomly slected. What is the probability that the visitor visits at least 1 of the 2 sites?
I tried adding 40%+70%+20% but that's more than 100%
Yes. What you did was count first with duplicates, and then subtract the duplicates to get the correct answer. (Well done!) Another (longer) way to think of it is:
40% visit mountains
70% visit parks
20% visit both
If 40% visit mountains and 20% visit both, then 20% visit ONLY mountains.
If 70% visit parks and 20% visit both, then 50% visit ONLY parks.
So (only mountains) + (both mountains and parks) + (only parks) = 20% + 20% + 50% = 90%.
By the way, the way you dealt with duplicates is a special case of the more general inclusion–exclusion principle.