How long Michael Phelps can cover distance of one side at a still lake (no current).

Question

Michael Phelps , the famous swimmer can swim downstream in a lake in exact 40 minutes (with the lake current helping time).
He can swim upstream in a lake in exact 60 minutes (against the lake current).
The length of lake is 2 kilometers.

How long Michael Phelps can cover distance of one side at a still lake (no current).

Answer 1

Speed = Distance / time
Speed downstream = 2KM / (40/60) Hr = 3 KM/Hr = Speed of the swimmer + Speed of the current ------- 1
Speed Upstream = 2KM / Hr = Speed of the swimmer - Speed of the current ------- 2

1 + 2

Speed of the Swimmer = 2.5 KM/ Hr

2.5 KM --------------------- 1 Hr
2 KM --------------------- ?

2/2.5 = 4/5 Hr = 48 Minutes.

Answer 2

Let m be Michael's speed and c be the speed of the current. Then:
2/m+c=40/60=2/3 hr
2/m-c=60/60=1 hr
So:
2m+2c=6
m-c=2
2m-2c=4
4m=10
m=10/4 kph as Michael's speed with no current
2/(10/4)=8/10=4/5 hour to cross the lake with no current
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