Figuring out distance and speed

Question

If a person rows to his favorite fishing spot

21 miles downstream in the same amount of time that he rows 7 miles upstream and if the current is

7 mph, find how long it takes him to cover 28 miles.

Answer 1

21 = t*(r+7) = rt+ 7t

7 = t *(r-7) = rt - 7t

28 = 2rt

14 = rt

21 = rt + 7t

21 = 14 + 7t

7 = 7t

t=1

21 = (r+7)*1 = r+7

r = 14

the speed of the boat is 14 mph. With the current (downstream), the speed is 14+7 = 21 mph, so 21 miles is traveled in 1 hour. Against the current (upstream) the speed is 14-7 = 7 mph, which means 7 miles is traveled in the same 1 hour time frame.

28 miles downstream requires 28/21 = 4/3 hours , or 1 hour and 20 minutes.

28 miles upstream requires 28/7 = 4 hours