I have a two family home. The downstairs bathroom has a plumbing problem. When the bathroom sink is used, the bathtub fills up with black water. What causes this and how do I fix it?

You have a blockage in the tub drain line that needs to be snaked out. You can snake the line from one of two different places. You can remove the overflow plate from the front of the tub and snake from there or you can snake from the lavatory roof vent. If you snake from the vent put out enough cable to reach the base and about 20 feet more. Good luck, Tom