Simplification for what we need to do here: There's one ship that takes x hours to complete a 400-mile journey, another takes x-2.5 less or 2.5 hours less than the other.
There are values we can test but with these kinds of questions, I reckon it's better to figure out a pattern that'll help lead us to the answer.
Let's take a small number. If one ship completes the journey in 5 hours, and the other will take 2.5 hours, with that, one is twice as fast as the other. Now, if one ship completes the journey in 10 hours, and the other completes it in 7.5 hours, the faster ship is 25% faster. The longer you add to the journey, the lower the percentage difference in the time covered.
Judging by this information itself, we have enough to eliminate a few answers - 80 miles / hour, which will allow one of the ships to complete the journey in 5 hours, doesn't have an equivalent for the other ship's that either good for 2.5 hours (160mph) or 7.5 hours (53.33mph). Clearly, no speed related to 80 miles / hour will feature anymore.
Is it the same for half that speed? 40mph will help one ship complete the journey in 10 hours, but we already know from above that at 7.5 hours, the ship'll need to be travelling at 53.33mph for that to work, which isn't an answer.
At 12.5 hours, we find the golden answer - 400 / 12.5 = 32 mph, and we can go ahead and mark the 32mph for Ship Alpha, and 40mph for Ship Beta.Now, isn't this the same as just taking the answers, and solving them? No, because there's enough process of elimination to help in our favor.
- 25mph cannot be the speed of the faster ship.
- At 64mph, we see a ship take 6.25 hours to complete the journey, and any 2.5-hour addition or subtraction - leading to 8.75 or 3.75 - clearly won't connect to any of the other numbers.
- 50 won't correlate to 64 and 80 because they've been eliminate. So if we take that to be the speed of the faster ship, the only 2.5 hours slower can't logically take double the time, so 25 is out of the window. 40 is easy mental math to discard - 8 hours vs 10 hours; that's a just 2-hour difference.
- With maybe a few more math tricks, 32 and 40 can be easily reached.
PS.
I also see value in looking at the factorization for 400, but while considering .5 numbers:
0.5*800
1*400
2*200
2.5*160
4*100
5*80
8*50
10*40
12.5*32
16*25
20*20
Clearly, we see the role of 32, and that can further help narrow the choices.
Bunuel