Question

For anti-ballistic missile system, the time of
flight tf is determined by the initial speed v0
of the missile and the range R.
What is the time (in minute) of the flight tf
available to kill an incoming intercontinental
ballistic missile launched from 10500 km away
with a speed of 8.8 km/s?

Answer 1

Assume you have the entire flight time of the ICBM available to make the kill. Make a flat earth, low trajectory approximation, with constant velocity of 8.8 km/s between launch and target.

The answer becomes simply 10,500/8.8 = 1193 s = 19.9 minutes.