Given cuboids of dimensions 15cm, 30cm ,15cm

LCM(15, 30, 15) = 30

Since 15*2 = 30

        30*1 = 30

and  15*2  = 30  {since a cube has all the dimensions are same}

So, the number of cuboids required to form a cube = 2*1*2 = 4

So, by using 4 cuboids, we can form a cube having side 30 cm each.