A farmer covers 61 km in 9 hours, partly on foot at 4 km/h and partly by bicycle at 9 km/h. What distance was covered on foot?

Let x be the distance (km) covered on foot at 4 km/h. Then (61 − x) km is by bicycle at 9 km/h. Total time is 9 hours.

Step-by-step calculation
Time on foot = x / 4Time on bicycle = (61 − x) / 9Total time = x / 4 + (61 − x) / 9 = 9Multiply by 36: 9x + 4(61 − x) = 3249x + 244 − 4x = 3245x = 80x = 16 km

Verification
Time on foot = 16 / 4 = 4 hTime on bicycle = 45 / 9 = 5 hTotal = 9 h (matches)

Common pitfalls
Avoid mixing distances with speeds; write times explicitly before summing.

Final Answer
16 km