Question

I need a big help into this math problem:
A boardwalk is parallel to and 210 ft inland from a straight shoreline. A sandy beach lies between the boardwalk and the shoreline. A man is standing on the boardwalk, exactly 750 ft across the sand from his beach umbrella, which is right at the shoreline. The man can run at a rate of 12 ft/sec on the boardwalk and 5 ft/sec on the sand. how far should he run on the boardwalk before veering of onto the sand if he wishes to reach his umbrella, and what is the minimum amount of time (nearest to the second) it will take him to reach that?