WHoa! As Trazor's answer shows, as long as it's understood, all that algebra just isn't needed. I noticed that one chap ("Softus") and maybe others didn't "get it" in the original thread. (The original thread has a link to the extended algebraic answer.)
You can use speed ratios with the total distance travelled, the distance after the first crossing, yada yada....... then facepalm when you see you didn't need it.
People tend to shut up when they don't follow. Maybe a couple of extra comments will help those:
First, there's more than one way to work out what the boats did, from the question - it needs to be drawn out. If you assume boats leave the other sides of the river it's an unhelpful detour!
Second, forget about time, after knowing that during the first crossing, the slow boat did 650m, and the fast boat did the rest. Yes you
can work out the ratios of the speeds of the boats if you want to and use it to work out what must have happened, but you can just draw it instead. The ten minutes' stop is a distractor. They both stop for once for 10 minutes, so it doesn't enter the equation.
Not to scale:
You're given the 650 and the 350.
The "Three crossings" thing is now obvious, that
BETWEEN THEM the two boats have done three crossings.
They have NOT done three crossings each.
You know that in the first crossing (of the two boats between them) the slow one did 650m.
So for each of the crossings, the slow boat's "contribution" if you like, is 650m
The "solution" is misleading if you read it as saying the slow boat has done three crossings; it hasn't.
You can say that during the three crossings, the slow boat's contributions must have been 3 x 650m, = 1950.
Then look at the pic to see that by itself it's actually done one crossing, plus 350m, so take the 350 off to get the 1600.
Then everything fits and you wonder what the fuss with the speeds and the distances and cancelling out of denominators....... was all about...