The volume of the vase when full is 12×12×45, but it’s only ⅔ full so the amount of water is ⅔×12×12×45 cubic centimetres (cc). The volume of each small container is 3×10×8 cc, so when we divide the volume of the water by the volume of one container, we get the number of containers that could be filled.
This number=⅔×12×12×45÷(3×10×8). We can do some cancelling: ⅔ of 12=8 which cancels with 8 in the denominator: 12×45÷(3×10)=4×9÷2=2×9=18. So we can fill 18 containers.