Grady rides his bike 60% faster than his little brother Noah. If Grady rides 15 miles further than Noah in three hours, how fast in miles per hour does Noah ride?

Let the distance covered by Noah==D

The distance covered by Grady ==D + 15

Let Noah's speed ==S

Grady's speed ==1.6S

3S == D.........................(1)

3 * 1.6S ==D + 15..........(2), solve for S, D

D==25 miles - distance covered by Noah

S==8 + 1/3 mph - Noah's speed