A person travels the first one-fourth of his journey time at 80 kmph ,...
Problem:
A person travels the first one-fourth of his journey time at 80 kmph ,and the remaining at 60 kmph .If he covers a total distance of 260 km ,find average speed over the entire journey?
Solution:
Step 1: Find the distance travelled at 80 kmph
Let us assume that the total distance travelled by the person is 'd'.
As per the problem, the person travels the first one-fourth of his journey time at 80 kmph.
Let the distance travelled at 80 kmph be 'd1'.
Therefore, d1 = (1/4) * d
Given, speed = 80 kmph and time = (1/4)th of the total time taken for the journey.
Using the formula, Speed = Distance / Time, we can calculate the time taken for the first one-fourth of the journey.
Time taken = Distance / Speed
Time taken = d1 / 80
Time taken for the remaining distance = Total time taken - Time taken for the first one-fourth of the journey.
Time taken for the remaining distance = (3/4)th of the total time taken for the journey.
Time taken for the remaining distance = (3/4) * Total time taken
Step 2: Find the distance travelled at 60 kmph
Let the distance travelled at 60 kmph be 'd2'.
Using the formula, Speed = Distance / Time, we can calculate the distance travelled at 60 kmph.
Speed = 60 kmph and Time = Time taken for the remaining distance.
60 = d2 / [(3/4) * Total time taken]
d2 = 60 * [(3/4) * Total time taken]
Therefore, Total distance travelled = d1 + d2 = (1/4)d + 60 * [(3/4) * Total time taken]
Step 3: Find the Total time taken
Using the formula, Speed = Distance / Time, we can calculate the Total time taken for the journey.
Speed = Total distance travelled / Total time taken
260 / Total time taken = (1/4)d / 80 + 60 * [(3/4) * Total time taken]
260 / Total time taken = (1/320)d + (45/2) * Total time taken
260 = (1/320)d * Total time taken + (45/2) * (Total time taken)^2
45(Total time taken)^2 - 520(Total time taken) + 640 = 0
Solving the above quadratic equation, we get Total time taken = 28/3 hours or 9.33 hours.