s law states that "the speed of recession of a galaxy is proportional to its distance from us."
Hence, If v is the speed with which a galaxy recedes and r is its distance from the earth,
or, v = H0r
where, H0 is called Hubble's constant. It's value is 2.3*10-18 s-1.
Significance of Hubble's law:
It is used to predict the age of the universe.
According to Hubble's law, matter at a distance r away from us is travelling with speed v=H0r.
The time t needed to travel a distance r is given by, t=r/v
= 4.3 * 10^17 s
= 1.4 *10^10 years
Hence we can conclude that the universe is 1.4 *10^10 years old.