Given:• The boy walks from home to school.
• If he walks at 5 kmph, he reaches 4 minutes early.
• If he walks at 4 kmph, he reaches 2 minutes late.
To find: The time (in minutes) he will take to reach school if he walks at 6 kmph.
Concept Clarification:Let the required time to reach school on time (without being early or late) be
T minutes.
Meaning:
• The boy starts at the right time to reach exactly on time in T minutes if he walks at the correct speed.
• If he walks
faster at 5 kmph, he covers the distance quicker and arrives 4 minutes early, meaning he takes
T – 4 minutes to reach.
• If he walks
slower at 4 kmph, he takes
T + 2 minutes to reach, so he arrives 2 minutes late.
Solution:Let the distance from home to school be
D km.
Then:
- At 5 kmph:
- Time = (T – 4)/60 hours
- Hence, D = 5 × (T – 4)/60 km
- At 4 kmph:
- Time = (T + 2)/60 hours
- Hence, D = 4 × (T + 2)/60
Since distance is the same in both cases, we have 15(T – 4) = 12(T + 2)
- 15T – 60 = 12T + 24
- 15T – 12T = 24 + 60
- 3T = 84
- T = 28 minutes
- So, D = 2 km (using any of the expressions for D)
- So, the scheduled time to reach school is 28 minutes.
- If he walks at 6 kmph, time taken = 2/6 hours
- = 1/3 hours = 20 minutes.
Correct Answer: (B)
Shweta KoshijaGMAT, GRE, SAT Coach for 10+ years