If someone could help me with this question I would be so grateful

Consider the Astroid belt as a torus surrounding the sun, populated by 1 million astroids, and with an astroid number density N = 2.3*10^-20 astroids / km^3. For an impact parameter S = 0.5km, and an assumed relative collision velocity in the belt of v0= 3km/s, determine the time between inter-asteroidal collisions. State all relevant equations clearly and define all terms.