Aircraft A 250 kts
Aircraft B 300 kts
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Speed difference: B is 50 kts faster than A.
A crosses the fix at 0630
B crosses the fix at 0636
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B is 6 minutes behind A.
At the time B crosses the fix A will have travelled 6 mins at 250 kts = 25 nm
When will B catch A? 25 nm to catch at 50 kts speed difference = 30 mins
Where will B catch A?
30 mins after A's 0636 position at 250 kts = 125 nm further on = 125 nm + 25 nm = 150 nm from the fix.
Check correctness:
30 mins after B's 0636 position at 300 kts = 150 nm from the fix which agrees with A's position above.