A boat can go 30 km downstream and 24 km upstream in 2 hours 27 minutes. Also, it can go 10 km downstream and 4 km upstream in 37 minutes. What is the speed of the boat downstream (km/h)?
Answer & explanation
Correct answer: option 2
D U
30 24 147 min
↓ ↓ ↓
10 4 37 min
↓ ↓ ↓
×3 ×3 ×3
↓ ↓ ↓
30 12 111 min
in upstream
12 km → in 147 - 111 = 36 min
1 km → 3 min
20 km → 60 min
U = 20 km/h
\(\frac{30}{D}\) + \(\frac{24}{20}\) = 147 min
\(\frac{30}{D}\) = 147 min - \(\frac{6}{5}\) × 60
\(\frac{30}{D}\) = 75 min
D = \(\frac{30}{75}\) × 60
D = 24 km/h