15/4 was about seeing if you noticed the error, completly bogus.
15 cm³ is 15 cc and 15 ml. 3375 cm³ is 15
³ cm³, as well as 3375 cc, etc. Basic maths/physics, if you take the cubic root of a magnitude, the units also go in the operation. ³√(3375 cm³) = 15 cm. You can never get cm³ from that.
If you want to write a "cube of 15 cm" (no ³ anywhere), fine, but if you ask for a "cube of 15 cm
³" to anybody doing physics properly, he will give you a tiny cube (a cube with sides of ~2.47cm).
By your logic, a 20 m³ shipping volume is a very big cube, when in reality if fits in a typical container.