Which of the following statements is correct?
I. A boat travels 30 km downstream in 6 hours. If she travels 40 km upstream in 10 hours, the speed of the stream is 1 km/hr.
II. The length of a train is 1200 meters. If it can cross a 1500 m long platform in 120 seconds, the speed of the train is 90 km/hr. 

Correct option is A

Given:
Statement I:
Downstream distance = 30 km, Time = 6 hours
Upstream distance = 40 km, Time = 10 hours
We are to check whether the speed of the stream = 1 km/hr
Statement II:
Train length = 1200 m
Platform length = 1500 m
Time to cross = 120 seconds
We are to check whether the speed of the train = 90 km/hr
Formula Used:
Speed = Distance / Time
For boats: Speed of stream = (Downstream speed − Upstream speed) / 2
For trains: Total distance = Length of train + Length of platform
Speed in km/hr = (Distance in meters / Time in seconds) × (18/5)
Solution:
Statement I:
Downstream speed = $$\frac{30}{6}$$​ = 5 km/hr

Upstream speed = $$\frac{40}{10}$$​ = 4 km/hr

Speed of stream = $$\frac{5-4}{2}$$​= 0.5 km/hr
The given statement I is incorrect (Statement says 1 km/hr, actual is 0.5 km/hr)
Statement II:
Total distance = 1200 + 1500 = 2700 meters
Speed (m/s) = $$\frac{2700} {120}$$​= 22.5 m/s
Speed (km/hr) = 22.5 × $$\frac{18}{5}$$​ = 81 km/hr
The given statement II is incorrect (Statement says 90 km/hr, actual is 81 km/hr)