Anyway, breaking it down (more detail than MATT's simplicity :) )
we need distance travelled by A = distance by B
therefore, d1 = d2;
d1 = s1t1, d2 = s2t2,
Let them both meet after time 't'.
so d1 = 6000*(20/60)+6000*t -> distance travelled by rocket A
and d2 = 8000*t -> distance travelled by rocket B
so 2000 + 6000t = 8000t1 (time since leaving would be same when at equal distance)
so 2000 = 2000t => t = 1 hour, so distance is 8000*1 = 8000 miles.
The key here is to make that important conclusion - "let them both meet after time 't' and then use d = st.
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