Question 366457
let speed of boat with no current be x mph
speed of current be y
..
downstream 
420/(x+y)=20
420 = 20x+20y
21=x+y.....................1
upstream 
420/  x-y=140
420 = 140(x-y)
420=140x-140y
3=10x-10y...................2
multiply equation 1 by 10
210=10x+10y
add this to equation 2
213=20x
x=213/20
x=10.65 mph ... speed of boat in still water
x+y =21
y=21-10.65
y= 9.35 mph ... speed of current 
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m.ananth@hotmail.ca