Let X be the distance traveled on foot. The distance traveled on the bicycle is then 90 - X km.
Since the time taken to cover the total distance is equal to the sum of the time taken to cover the distances partly on foot and partly on the bicycle, we can set up the following equation: X/9 + (110 - X)/15 = 10
Solving for X, we get: 5X + 3(110 - X) = 45 * 10
5X + 330 - 3X = 450
2X = 450 - 330
2X = 120
X = 120/2 = 60
Therefore, the person traveled 60 km on foot.