Answer:
1.49 hrs or 1 hr and 30 minutes approximately
Step-by-step explanation:
if fran go 2.8m up then the river flow at a rate of 0.8m; the since we are taking the speed of fran and river to be constant. the ratio between their speed will always remain constant. that is if 0.8/2.8 = 0.286≈ this will be always constant.
so if he go up 4m then the horizontal length has to be 1.143 because 1.143/4 = 0.286.
now we find the distance between frans initial positon and final position. to find it we use pythagoras theorem.
to find the time divide distance traveled by speed;
4.160/2.8 = 1.49 hrs