collision avoidance calculation

Question 1

With a flight visibility of 3nm, a light aircraft and a military jet have a closing speed of 400 kts. Approximately how much time do the pilots have to avoid a collision if visual contact was made at maximum range?

Answer 1

Closure speed = 400kts
Closures speed/ 60

we know 1nm = 60 knot
Xnm= 400 knots
= 400/60
=6.6nm

6.6m = 60 seconds
3m = X

3x60/6.6 = 27 seconds