Put 6 cm along 3 dm = 30 cm 5 times. Put 4 cm along 4 dm = 40 cm 10 times. Put 10 cm along 0.5 m = 50 cm 5 times. The number is 5*10*5 = 250. It will be no remaining empty space in the box. It means that we put maximal possible number of pieces of soap into the box.