Show that at 40km/h it takes 1 minute and 30 seconds to travel 1km

Let's compare apples to apples and oranges to oranges.  This means converting seconds to hours or hours to seconds:  3600 sec = 1 hour.

40 km/h = 40 km/h times (1 hour) / (3600 sec) =  (1/90) km/sec

Then the time req'd to travel 1 km is:

distance                     1 km
= time, or   = 90 sec = 1 1/2 minutes
    rate                         1 km
                                   
                                    90 sec