A man is walking at the rate of 5 km/hr crosses a bridge in 15 minutes. The length of the bridge is

Answer: Option C

Explanation:

Let time taken to travel the first half = x hr
Then time taken to travel the second half = (10 - x) hr

Distance covered in the the first half = 21x [because, distance = time*speed]
Distance covered in the the second half = 24(10 - x)

Distance covered in the the first half = Distance covered in the the second half
So,
21x = 24(10 - x)
=> 45x = 240
=> x = 16/3

Total Distance = 2*21(16/3) = 224 Km [multiplied by 2 as 21x was distance of half way]

Answer: Option D

Explanation:

$$\begin{aligned} Speed = \frac{Distance}{Time} \\ = \frac{200}{24}m/sec = \frac{25}{3} m/sec \\ \frac{25}{3}*\frac{18}{5} km/hr = 30 km/hr \end{aligned}$$

Answer: Option A

Explanation:

We know that,
$$\begin{aligned} Time = \frac{Distance}{Speed} \\ Speed = 20\text{ km/hr} = 20*\frac{5}{18}{ m/sec} \\ = \frac{50}{9}{ m/sec} \\ \text{ Time =} \left(400*\frac{9}{50}\right) \\ = 72 {sec} = 1\frac{1}{5}{ min} \end{aligned}$$

Answer: Option A

Explanation:

We know total distance is 200 Km
If both trains crossed each other at a distance of 110 km then one train covered 110 km and other 90 km [110+90=200km]
So ratio of their speed = 110:90 = 11:9

Answer: Option B

Explanation:

We are having time and speed given, so first we will calculate the distance. Then we can get new speed for given time and distance.
Lets solve it.

Time = 50/60 hr = 5/6 hr
Speed = 48 mph
Distance = S*T = 48 * 5/6 = 40 km

New time will be 40 minutes so,
Time = 40/60 hr = 2/3 hr
Now we know,
Speed = Distance/Time
New speed = 40*3/2 kmph = 60kmph