In 6 hours, the first jellyfish will have swum 4 3/4 hours.
In 6 hours, the second jellyfish will have swum 6 hours.
Together, they will have swum 4 3/4 + 6 = 10 3/4 hours in 6 hours.
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Actually, it depends on how much time they swim and rest in the 6-hour interval you're counting. Here, we have assumed that the interval of observation overlaps completely the times the jellyfish spend swimming, and that the second jellyfish does not rest during that time.
