Guided Ganges

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Two evening guided boat tours are traveling toward each other on the Ganges, one at rate X and the other at 3X. At what distance will they collide. (Show your work for partial credit.)

Mouse over the image for an alternate post-processed version. The difference is subtle but in the end, it was difficult to decide on only one.

EDIT (Mar. 30, 2009): I've changed the rollover image to a flash jQuery animation. Mouse over the image to change versions.