d = 5,000 mi the distance between the missiles
v1 = 2,000 mph the rate of the first missile
v2 = 1,000 mph the rate of the second missile
t = 1 min = 1/60 hr
Since distance = speed*time the distance 1 minute before the impact is:
v1t + v2t = t(v1 + v2) = (1/60)(2,000 + 1,000) = 50 miles.