On the way there Kevin drove 90 miles, and on the way back drove the (total distance - 100 miles).
On the way, Daniel drove the (total distance - 90 miles), and on the way back drove 100 miles.
So Kevin drove 90 + total - 100 = total - 10.
And Daniel drove total - 90 + 100 = total + 10.
So Daniel drove 20 miles more than Kevin.
Hope this helped you