According to the meaning of the question;

y+z=30 ①

x+y=28 ②

x+z=24 ③

②-① De: x-z=-2。 ④

③+④: 2 x = 22。 De: x= 1 1。

Substitute x= 1 1 into ②:

y= 17。

Substitute y= 17 into ①:

z= 13

So x = 1 1, y = 17, z = 13.