Concept: The question tests on the basics of Rates(Time and Work)

Solution:

Using the concept of Man Days, where M1T1 = M2T2

Initial Number of Men = x

Initial days = 10

New Number of men = x - 5

New Number of days = 12

Therefore 10x = 12(x - 5)

10x = 12x - 60

2x = 60

x = 30

Option C