I tried to solve this question with algebra while doing the test. It has since been reworded:
A boat crossed a lake from North to South at the speed of 4 km/h, entered a river and covered twice as much distance going upstream at 3 km/h. It then turned around and stopped at the south shore of the lake. If it averaged 3.8 km/h that day, what was its approximate downstream speed?
The answer choices and numbers remain the same.
So d1 (distance crossing the lake) = 4t1 (where t1 = time spent cross the lake)
d2 (going upstream) = 3 * t2 = 2 * d1 = 8 * t1
d3 = d2 = s * t3 (where s is the downstream speed) = 2 * d1 = 8 * t1
So average = 3.8 = t1(4 + 8 + 8)
t1 + 8/3*t1 + t3
Solve and you get something like
18.2 * t1/3 = 3.8 * t3
t3 = (8 * t1)/s
substitute and you can get s = (3.8 * 8 * 3) / 18.2
IMO numbers were hard to manipulate. Thats why I got it wrong