Let:
* A’s speed = a
* B’s speed = b
They start at same time and meet after t minutes.
STEP 1: Distances before meeting
Before meeting:
A travels:
at
B travels:
bt
STEP 2: Use the “swapped remaining distances” idea
After meeting:
* A still has to travel the part B already covered before meeting.
* B still has to travel the part A already covered before meeting.
So:
A after meeting:
A takes 40 min after meeting.
Distance traveled by A after meeting:
40a
But this equals B’s pre-meeting distance:
bt
So:
40a = bt
B after meeting:
B takes 90 min after meeting.
Distance traveled by B after meeting:
90b
But this equals A’s pre-meeting distance:
at
So:
90b = at
Now we have:
40a = bt
90b = at
STEP 3: Find speed ratio
Divide the two equations:
40a / 90b = bt / at
Simplify right side:
bt / at = b / a
So:
40a / 90b = b / a
Simplify left side:
4a / 9b = b / a
Cross multiply:
4a2 = 9b2
Take square root:
a / b = 3 / 2
So:
A speed : B speed = 3 : 2
STEP 4: Use speed ratio to get meeting time
Since speeds are 3 : 2,
before meeting their distances are also 3 : 2
(because they traveled for same time before meeting).
Suppose:
a = 3k
b = 2k
Using:
40a = bt
Substitute:
40(3k) = 2k(t)
120k = 2kt
Cancel 2k:
t = 60
So they met after 60 minutes.
STEP 5: Find A’s total travel time
A traveled:
* 60 min before meeting
* 40 min after meeting
Total:
60 + 40 = 100 minutes
= 1 hour 40 minutes
Answer: D