SSC CHSL 6 Dec 2015 Shift 2 Paper

L.C.M. (12,18,21) = 252
Lowest five digit number = 10000
Now on dividing 10000 by 252, remainder = 10000%252 = 172
Thus, smallest five digit number which is divisible by 12, 18 and 21 = 1000 + (252 − 172) = 10080