Loaded test-tube placed in pure milk sinks to a certain mark (M). Now some water is mixed with the milk. Will the test tube sink more or less? Explain.

When water is added, the density of milk decreases, so the weight of milk-water mixture displaced by immersed part of the tube also decreases and becomes less than the weight of loaded tube. So the tube sinks more and will float only when the weight of this milk-water mixture displaced by the tube is equal to the weight of loaded tube.