Question

A piece of rubber has a mass of 495 kg and a volume of 355 liters. Will it float in water, which has a density of 1.0 g/mL?

Answer 1

Compare the density of the rubber to water. If it is less that 1 g/mL then it floats in the water.

You must compare density in the same units. So convert to g/mL 

$( \frac{495kg}{355L} )*( \frac{1000g}{1kg} )*( \frac{1L}{1000mL} ) = 1.39 g/mL$

The rubber does not float in water.