Let the velocity of water be XM/min；; The time for boating enthusiasts to turn around when their hats fall into the sewer is t; The time from turning around to picking up the hat is t "; T from the time the hat fell into the water until it was picked up.

Then (10-x)*t=80.

Xt=20

x = 2m/min； T = 10 minutes

t"*( 10+2)=80+2t "

T "= minutes

T = t+t "= 10+8 = 18 minutes.