Answer :
20% of them order neither.
The total percentage of donuts ordered is 70%; this includes the 40% that order both. This leaves 70-40 = 30% that order only donuts.
The total percentage of cinnamon rolls ordered is 50%; this includes the 40% that order both. This leaves 50-40 = 10% that order only donuts.
This leaves 100-30-40-10 = 20% that do not order either.
The total percentage of donuts ordered is 70%; this includes the 40% that order both. This leaves 70-40 = 30% that order only donuts.
The total percentage of cinnamon rolls ordered is 50%; this includes the 40% that order both. This leaves 50-40 = 10% that order only donuts.
This leaves 100-30-40-10 = 20% that do not order either.