Final answer:
The density of a pebble with a mass of 1.40g that displaces water in a tub from 1980 mL to 2020 mL is calculated as 0.035 g/cm³.
Step-by-step explanation:
When a pebble with a mass of 1.40g is placed in a tub of water, causing the water level to rise from 1980 mL to 2020 mL, the volume of water displaced by the pebble is the difference in these two measurements. Therefore, the volume displaced is 2020 mL - 1980 mL = 40 mL, which is also the volume of the pebble (1 mL of water equates to a volume of 1 cm³).
The density of an object is calculated by taking its mass and dividing it by its volume. In this case, the mass of the pebble is 1.40g and the volume is 40mL or 40cm³. Using the formula density = mass / volume, we get density = 1.40g / 40cm³ = 0.035 g/cm³.