Aswimmer heading directly across a river 200 m wide reaches the opposite bank in 6 min 40 s, during which time she is swept downstream 480 m. (a) how fast can she swim in still water?

 Swimmer can swim at a speed of 0.5 m/s in still water.

Explanation:

 Since the swimmer is heading directly across a river 200 m wide, her horizontal component of velocity is zero, she only got vertical component and which is taking her to the opposite direction. So the vertical component of her velocity is the  velocity with which she can swim in still water.

Vertical displacement = Width of river = 200 m

Time taken  = 6 min 40 s = 400 seconds.

So velocity of swimmer in still water = 200/400 = 0.5 m/s

a.   mass number from atomic number