Answer:
The law of conservation of mass
Step-by-step explanation:
In physics and chemistry, the law of conservation of mass or principle of mass conservation states that for any system closed to all transfers of matter and energy, the mass of the system must remain constant over time, as the system's mass cannot change, so the quantity can neither be added nor be removed.
rho = density
t = time
nabla = divergence
v = flow velocity field