Here we need to recognise that the speed is in Kilometers per hour but the distance is in miles.
First job is to make them use the same measure so lets convert 50 kph to miles per hour (mph)
Looking it up on Google one km is 0.621371miles, so 50 kph is .62137 times 50 = 31.0686 mph
Now we can say we will cover 31.0686 miles every hour so 18 miles will take lots less than an hour ... it will take 18 divided by 31.0686 hours
ie 18 /31.0686 = .579363 hours (and lots more decimal places) or nearly 35 minutes (.579363 times 60=34.76 mnutes or nearly 34 minutes 23 seconds! )