78. The Hope Primary School is 48 km away from the county town, part of which is an uphill road, and the rest is a downhill road. Principal Zhang rides a bicycle from the school to the county town, taking 4.2 hours to go and 3.8 hours to return. It is known that Principal Zhang rides a bicycle uphill at a speed of 10 km per hour, so he rides a bicycle downhill at a speed of $\qquad$ km per hour.
Reference answer: 15