If I were attempting to solve it, I’d first (roughly) convert km to nautical miles to determine separation at “maximum visual range,” since the closure rate is given in knots. 4 km is roughly 2 nautical miles. Then solve for closure rate. 280 knots is roughly 4.7 nautical miles per minute. What’s the closest correct answer? BTW, where did you find this question?

I am going to guess Canada because of the use of metric system and worded in English.. Two other clues that this is not a USA FAA exam question is there are four possible answers instead of three, and there are small case answer choices (a., b., c., d.) instead of upper case (A., B., C.)

1 nautical mile is about 1.85 km. So, 4 (km)/1.85 (km/NM)= 2.16 NM. They are about 2.16 NM apart at the initial visual contact. Knots is NM/h. Thus, they will collide in 2.16 (NM) / 280 (NM/h) hours, which is 0.0077 hours. Therefore, 0.0077*3600 = 27.8 seconds.

TROLL...….. just getting that out of the way, lol. That is a PIA question, you have two different unit of measures used, so you have to convert one to the other. I'll convert km to nm, using the googles, 4km = 2.16nm, then I would divide 2.16 nm by 280 nm/hour which would equal, 0.008 which is actually 0.008 hours. Now you need to convert that to seconds, so knowing there are 3,600 seconds/1 hour I would multiply 0.008 hours times 3600 sec/hour which equals 29 seconds. So A is correct.... (I rounded some numbers) Or I'm sure someone sells a whiz wheel that would figure it out for you.