The Sinhagad Express left Pune at noon sharp. Two hours later, the Dec...
The ratio of time for the travel is 4:3 (Sinhagad to Deccan Queen). Hence, the ratio of speeds
would be 3:4. Since, the sum of their average speeds is 70 kmph, their respective speeds would
be 30 and 40 kmph respectively. Use alligation to get the answer as 34.28 kmph.
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The Sinhagad Express left Pune at noon sharp. Two hours later, the Dec...
Given data:
- Sinhagad Express started from Pune at noon sharp.
- Deccan Queen started from Pune in the same direction two hours later.
- Deccan Queen overtook Sinhagad Express at 8 p.m.
To find:
- Average speed of the two trains over this journey.
Approach:
- Let the average speed of Sinhagad Express be x km/h.
- Therefore, the average speed of Deccan Queen will be (70 - x) km/h (given sum of their average speeds is 70 km/h).
- Let the distance travelled by Sinhagad Express in 8 hours (from noon to 8 p.m.) be d1 km.
- Therefore, the distance travelled by Deccan Queen in 6 hours (from 2 p.m. to 8 p.m.) will also be d1 km (because it overtook Sinhagad Express).
- Let the total distance between Pune and the point where Deccan Queen overtook Sinhagad Express be d km.
- Therefore, the distance travelled by Sinhagad Express in the first 6 hours (from noon to 6 p.m.) will be (d - d1) km.
- Now, we can use the formula: distance = speed × time to form two equations and solve them simultaneously to get the value of x.
Calculation:
- Distance travelled by Sinhagad Express in the first 6 hours = (d - d1) km
- Time taken by Sinhagad Express to cover this distance = 6 hours
- Therefore, x = (d - d1) / 6
- Distance travelled by Deccan Queen in 6 hours = d1 km
- Time taken by Deccan Queen to cover this distance = 6 hours
- Therefore, (70 - x) = d1 / 6
- Adding the above two equations, we get:
70 = d / 6
d = 420 km
- Substituting the value of d in the above equations, we get:
x = 28 km/h
(70 - x) = 42 km/h
- Therefore, the average speed of the two trains over this journey will be:
(2xd) / (8 hours) = (2 × 420) / 8 = 84 / 4 = 21 km/h
- But we need to find the average speed of the two trains together, which will be:
(average speed of Sinhagad Express + average speed of Deccan Queen) / 2
= (28 + 42) / 2 = 70 / 2 = 35 km/h
Therefore, option (A) 34.28 km/h is incorrect and the correct answer is option (B) 35 km/h.
The Sinhagad Express left Pune at noon sharp. Two hours later, the Dec...
How this 240 comes