Hey could someone help me solve this question?

Two trains travelling at speeds of 36 kmph and 54 kmph cross each other in 8 seconds when travellling in opposite directions. Had they been travelling in same direction, in how much time would the faster train have crossed the slower train?

Speed of first train = 36 km/h = 10 m/s

Speed of second train = 54 km/h = 15 m/s

Time to cross in opposite directions = 8 seconds
Relative speed (opposite directions):
10
+
15
=
25
 
m/s
10+15=25m/s

Total length of both trains:
25
×
8
=
200
 
m
25×8=200m

Relative speed (same direction):
15
−
10
=
5
 
m/s
15−10=5m/s

Time to cross in same direction:
200
5
=
40
 
seconds
5
200
​

=40seconds