Don't Pay the Ferryman!

Until you figure out how wide the water is. This is a dynamicization of a puzzle found at the Futility Closet. http://www.futilitycloset.com/2014/01/30/in-a-word-454/ It sets up with random but reasonable numbers each time. I probably should have set it up as ferries to Mackinac Island with Arnold's and Shepler's ferries, but maybe for version 2. I love these algebra problems that look like they don't have enough information. This reminds me of NRICH's burning candle problem (http://www.geogebratube.org/material/show/id/10226) or the Escalator problem (http://www.geogebratube.org/material/show/id/14703).