Question 911413
r*t=d
with the  current
(r+c)*4=32
(r+c)=32/4
r+c=8
against the  current
(r-c)*4=16
(r-c)=16/4

r-c=4
r+c=8
add
r+r=4+8
2r=12
r=6
r=6 boat mph

6-c=4
c=6-4
c=2 mph current
check
(6+2)*4=32
8*4=32
32=32
ok
(r-c)*4=16
(6-2)*4=16
4*4=16
16=16
ok