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See diagram.
So it takes 11 seconds for the crab to touch the 5 cm point from the start.

At the end of 3 seconds, the crab advances by 1 cm. At the end of 9 seconds, the crab advances to 3 cms. Now in the next two seconds , the crab crawls 2 cms. It touches the 5 cm point. So at 11th second, it touches 5 cm line.

But, of course, in the next second, i.e., it crawls back by 1 cm.