Deepa covers 61 km in 9 hours, walking part of the way at 4 km/h and cycling the rest at 9 km/h. The distance she walked (in km) is:

If she walked d km then d/4 + (61 − d)/9 = 9; multiplying by 36 gives 9d + 244 − 4d = 324, so d = 16 km (4 hours walking, 45 km cycled in 5 hours). The 45 km is the distance CYCLED, not walked, and 18 km is what you get by assuming she spent half the 9 hours walking; 36 km assumes she walked the whole 9 hours.