In a hostel of 50 students, 36 have a laptop and 29 have a tablet. What is the least number of students who must have both?

Explanation

1. 01

   The overlap is smallest when as many students as possible have at least one device, that is, all 50.

2. 02

   Then both $= 36 + 29 - 50 = 15$.

3. 03

   A smaller overlap would need more than 50 students, so at least 15 have both.