A person takes 20 minutes more to cover a certain distance by decreasing his speed by 20%. What is the time taken to cover the distance at his original speed?

Question: A person takes 20 minutes more to cover a certain distance by decreasing his speed by 20%. What is the time taken to cover the distance at his original speed?

Solution:
Let the distance and original speed be 'd' km and 'k' kmph respectively.
d/0.8k - d/k = 20/60
⇒ 5d/4k - d/k = 1/3
⇒ (5d - 4d)/4k = 1/3
⇒ d = (4/3)k

Time taken to cover the distance at original speed
= d/k = 4/3 hours = 1 hour 20 minutes