Question 1149799
d=v*t
63=(15+c)*t where c is current speed
t=63/(15+c)
and 
63=(15-c)(t+4)
and  t=[63/(15-c)]-4, which is (4c+3)/(15-c), from putting -4 over 15-c and getting -60+4c
those t s are equal
cross-multiply and get 2c^2+63c-450=0
and this is (2c+75)(c-6)=0
positive root is c=6 kph current

this would be 21 kph downstream and 3 hours to do 63 km and 9 kph upstream, which takes 7 hours.