Of 100 students in a Pune hostel, 80 have a phone, 70 have a laptop and 60 have a bicycle, as the note says. At least how many students must have all three?

Students missing at least one item number at most $20+30+40=90$ (20 without a phone, 30 without a laptop, 40 without a bicycle). So at least $100-90=10$ have all three. 0 assumes nobody has to own all three, 50 is the least number owning both a phone and a laptop $(80+70-100)$, and 60 is the largest possible number.