Question 910265
r*t=d
with the  current
(r+c)*4=640
(r+c)=640/4
r+c=160
against the  current
(r-c)*4=240
(r-c)=240/4

r-c=60
r+c=160
add
r+r=60+160
2r=220
r=110
r=110 boat

110-c=60
c=110-60
c=50 current
check
(110+50)*4=640
160*4=640
640=640
ok
(r-c)*4=240
(110-50)*4=240
60*4=240
240=240
ok