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

it's when something is going so fast it makes a loud noise, breaking the sound barrier, for example: a jet makes a loud noise, not only cause of the jet but because they reached a point where they went so fast that you can year the noise from various parts, kinda like a police car, lets say the car was going   so fast that you hear the siren of the cop car right next to you, but the car is probably a mile a way, it's cos the siren was next to you a few seconds if you understand me

i dont have a visual in front of me, but i can you reason to try to get the right one. the graph should look like this, 0 to 6 there is a spike, 12 to 14 there should be a very low point. slight increase in the graph at 12 to 14, and flat 14 to 18

explanation: