Add up the people who answered both questions correctly and subtract the total number of people, and the extra change is the number of people who answered both questions correctly.

If someone does both questions wrong, there can be at most 10 (because the first question is only 10, and it is impossible to exceed 10), that is, 18 people are right in both questions, and12-/kloc-0 = 2 people.

So the number of people who answer both questions correctly is between 4 and 10.