Awater tank has a mass of 3.6 kg
when it is empty. the same tank
has a mass of 51.5 kg when it is
full of water. what is the mass of
the water when the tank is full?

The mass of the water is 47.9 Kg

Explanation:

Adding and subtracting masses

The mass of an object, like many other physical magnitudes, can be added or subtracted to get resultant objects with more or less mass. Since mass is a scalar magnitude, such operations don't involve the use of vectors, just arithmetics.

We know the water tank has a mass of 3.6 kg when it's empty and we also know that it has a mass of 51.5 kg when it is full of water. The difference between both quantities must necessarily be the mass of the water, so  

Mass of the water=51.5 Kg - 3.6 Kg = 47.9 Kg

The mass of the water is 47.9 Kg

i don't understand. explain it clearly

it would be c. predictability