Question

What is the time 15 hours before 11 p.m.? (3 marks)

Answer 1

15$\equiv$11-x (mod 12) (1 mark)
15-11+x=12n for some integer n
4+x=12n
4+x is a multiple of 12 (1 mark)

Therefpre, the least positive value of x must be 8 Since 4+8=12 is multiple of 12. Hence the time 15 hours before 11 o'clock p.m. is 8p.m. (1 mark)