Where Is the Clog

[Coolbreeze]

Below is the layout of the drain lines in my home. One of the toilets started gurgling the other day and I had a heck of a time clearing the clog. When I plunged the toilet that was gurgling it was like there was no resistance in the line thus the plunging action was ineffective. It finally cleared after multiple (8 - 10) attempts. Any idea where the clog occurred?

 

[WorthFlorida]

It was past the vent for the toilet. As you pushed, water or air went up the vent pipe if the blockage was tight.