Question 472725
The speed formula is:  v=d/t

You have given 
speed from A to B  v1=81km/hr
speed from B to A v2=54km/hr
total travel time t1+t2=5hr
the distance d from A to B and B to A is the same
Then
v1=d/t1  d=v1*t1
v2=d/t2  d=v2*t2
since distnce is same you compare 2 equations

v1*t1=v2*t2
81*t1=54*t2
 Now use information about the time t1+t2=5hr ---> t1=5-t2 substitute and solve fo t2 
81*(5-t2)=54*t2
405-81t2=54*t2
135*t2=405
t2=3hr then trip by car t1=2hr