Transmission Line: Converting from impedance to admittance. Why?
The left half of the line is loaded by the right half in parallel with the 30Ohm resistor. In admittance domain you can simply add that resistor to the admittance of the right part by increasing the real part. The needed increment is 50/30 = 1,67 in relative units and you get the admittance of the right half in relative units from Smiths chart.

You can use the same chart for impedances and admittances (rotate 180 degr) and you can even calculate the admittance of ZL with the chart (=find the same reflection factor). Check this if, you didn't know it before: 

Example Admittance Calculations with the Smith Chart