Two ferries start to cross a river at the same time from opposite shores. They travel at constant speeds and pass 720 yards from one shore. They remain in thier docks for ten minutes and then return across the river at the same speeds. This time they pass 400 yards from the other shore. How wide is the river?
Comment: Devising a sequence of easier, but increasingly complex, problems that "converge" to this one might be an interesting challenge.
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Simplify
(x - a)(x - b)(x - c)...(x - z)
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