Question 311408
distance traveled downstream = 210 miles
Time taken = 10 hours
downstream speed = 210/10 = 21 mph
..
distance traveled upstream = 210 miles
Time taken = 70 hours
upstream speed = 210/70 = 3 mph


let speed of boat in still water be x mph
let speed of current be y mph
.
x+y=21
x-y=3
.
add the two equations
2x=24
x=12 speed of boat in still water
plug the value of x in any equation above
y=9 mph speed of current