Question

What is time 15 hours before 11 p.m.?

Answer 1

15 ≡ x (mod 12)

15 – x = 12n

15 – x is a multiple of 12 x must be 3.

∴ The time 15 hrs before 11 O’clock is 11 – 3 = 8 O’ clock i.e. 8 a.m.

Answer 2

Here, 

11 p.m. = 23 hours

∴ The time 15 hrs before 11 O’clock is 23 – 15 = 8 O’clock i.e., 8 a.m.