Shipments for a certain product originate at point X and are to be sent to points Y

Question:

Shipments for a certain product originate at point X and are to be sent to points Y and Z. Y is an intermediate point to X and Z. The rate to Y is $1.20 per cwt., but due to competitive conditions at Z, the rate to Z is $1.00 per cwt. Apply the principle of blanketing back, and explain how it eliminates rate discrimination.