The clog is below the point where the toilet and the tub join the drain.
Putting toilet water into the tub with a cup.The real answer isWater seeks its own level and when a soil line (waste) is blocked water will find the lowest point to flow intoThus a toilet is above the tub and a basin is above the toilet.To prove allow water to flow in the basin and it should also flow into the tub
Because the two pipes are connected, and the shower is farther down the line than the toilet. So when you flush, you are hearing the water from the toilet pass by your tub. This is also why you will have sewage in your tub if your waste water line is extremely backed up.
Iron? == == Possibly a slight blockage allowing toilet water to ernter the waste line of the tub
15 inches from outside edge of tub to center of toilet flange.
The toilet itself is plugged or the pipe from the toilet to the drain is plugged. It may be that the tub and sink have their own drain pipe that may connect to the main away from the toilet drain. Plunge the toilet or sometimes if you fill a 5 gallon bucket with water and pour it into the toilet as fast as it will take it, that will flush out the line. It is a greater amount of water all at once an forces out what the clog is.
If the waste from one toilet is backing into another toilet, sink, or tub, you may need to use a back- flow prevent-er. This will keep all drainage from your home flowing in the proper direction. Also, you could have a clogged drainage pipe between the two fixtures!
A bathroom would have a toilet-sink-tub and or shower. A half bath has a toilet maybe a sink but no tub or shower.
Water seeks its own level zand the tub and basin drain into the same waste line. The back flow into the tub means the drain has a partial stoppage down stream of the basin waste and the waste flows back into the tub as the tub waste is lower then the basins waste
People go in a tub then the tub empties it into the air.
The drain is plugged between the tub and the sewer or septic or in the case of the septic system, the tank may be full. The toilet had to go somewhere and the tub was available.
The tub and toilet connect to the same drain at some point. The main drain was plugged but the connection between the toilet and tub was still open. Water seeks it's own level. When the water came up in the toilet, the tub was lower so some of it went there.
You have to use toilet more