Answer:
 about 26 min
Explanation:
Let x represent the time spent rowing distance d upstream. Then the time spent rowing downstream is ...
 time = distance/speed . . . . speed × time = distance
 downstream time = (4.5 km/h)(x)/(5.5 km/h)
So, the total time in both directions is ...
 x + 4.5/5.5x = 48 min
 x(1 + 9/11) = 48 min
 x = (48 min)/(20/11) = 26.4 min
Rounded to the nearest minute, Toni spent about 26 minutes rowing upstream.