5x+8y+ 10z= 120……①，
x+y+z= 14……②
②* 10-① to obtain
5x+2y=20，
y=(20-5x)/2，
According to the meaning of the question, x, y and z are all natural numbers.
5x must be an even number less than 20,
X can only take 2.
When x=2 and y=5,
Substitute ② to get z=7.
400*2+500*5+600*7=7500 yuan.
The quantities of the three models are 2, 5 and 7 respectively, and the freight at this time is 7500 yuan.