The Question
"Airlines monitor the causes of flights arriving late. A total of 75% of flights are late because of weather, while 35% of flights are late because of ground operations. A full 15% of flights are late because of weather and ground operations. What is the probability that a flight arrives late because of weather or ground operations? (2 numbers behind decimal, example: 0.34)"
The Supposed Solution
My friend just did this: $0.75+0.35-0.15=0.95$
My Question
This may be an overcomplication, but I just feel that there is something wrong with this question. How is it possible for a portion, represented by percentages, to total larger than 100%? Is it just me or is the question mistaken?
There are no errors in the question. Look at this Venn representation of your problem
where the red circle is the event: late due to weather conditions and the blue one is the event: late for Ground operations
What is the probability that an airplane arrives late ONLY for weather conditions? Obviously it is not 75% because this probability does not exclude to have both bad weather conditions AND ground operations too...