A boat travels 2km upstream and 2km downstream. The total time of the trip is 1 hour. The speed of the stream is 2km/h. What is the speed of the boat in still water?

2 = (v + 2) * t1;

2 = (v - 2) * t2;

t1 + t2 = 1;

where v is the speed of the boat in still water; t1 is the time upstream; t2 is the tome downstream;

Then, v + 2 = 2 / t1 and v - 2 = 2 / t2;

v = 2 / t1 - 2 and v = 2 / t2 + 2;

2 / t1 - 2 = 2 / t2 + 2;

2 / t1 - 2 / t2 = 4;

1 / t1 - 1 / t2 = 2;

t2 - t1 = 2t1t2;

But, t2 = 1 - t1;

1 - t1 - t1 = 2t1 (1 - t1);

1 - 2t1 = 2t1 - 2t1^2;

2t1^2 - 4t1 + 1 = 0; quadratic equation;

solving this equation,

t1 ≈ 0.29 hour;

v ≈ 2 / 0.29 - 2;

v ≈ 4.89 km/h;