What will be the least number which when doubled will be exactly divisible by 12, 18, 21 and 30 ?

Answer: Option B

12 = 2 × 2 × 3
18 = 2 × 3 × 3
21 = 3 × 7
30 = 2 × 3 × 5

L.C.M. of 12, 18, 21 30 = 2 × 3 × 2 × 3 × 7 × 5 = 1260
Required number = $$\frac{1260}{2}$$  = 630