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

 L.C.M. of 12, 18, 21 30                 2 | 12  -  18  -  21  -  30
                                         ----------------------------
   = 2 x 3 x 2 x 3 x 7 x 5 = 1260.       3 |  6  -   9  -  21  -  15
                                         ----------------------------
   Required number = (1260 ? 2)            |  2  -   3  -   7  -   5

                   = 630.