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