First of all you need to work out 20 minutes in hours, this can be done by dividing 20 by 60:
20/60 = 1/3
Then you need to divide 15 by 1/3 to get the final speed:
15 / 1/3 = 45 miles per hour
He will need to be traveling at 45 miles per hour,
Hope this helps! :)
