Solution: suppose Zhang Ming goes out at 6: 00 pm for x minutes, then there is

6*30-5.5x= 135

Solution, x = 90/11;

Suppose Zhang Ming comes back at 6: 00 p.m. Y, then there will be

5.5y-6*30= 135

Solution, y=630/ 1 1.

So the time for Zhang Ming to go out is y-x = (630-90)/1= 540/11(a little over 49 minutes).