A cyclist has to cover a distance of 80 km. After cycling for 4 hours and 48 minutes, he finds that he has completed 4⁄5th of the total distance. What is his speed in km/h?
- (a)15.45 km/h
- (b)16 km/h
- (c)13.33 km/h
- (d)10 km/h
Answer
Why
Correct — C.
Distance done: 4⁄5 × 80 = 64 km
Time in hours: 48 min = 48⁄60 = 0.8 h, so 4 h 48 min = 4.8 h
Speed = distance ÷ time = 64 ÷ 4.8
= 640 ÷ 48 = 40⁄3 ≈ 13.33 km/h → option (c)
Why the others are wrong
- (a)15.45 km/h — 15.45 km/h for 4.8 hours covers 15.45 × 4.8 ≈ 74.2 km, not the 64 km (4⁄5 of 80) actually ridden.
- (b)16 km/h — 16 km/h is 64 ÷ 4: it drops the 48 minutes. Over the full 4.8 hours, 16 km/h would cover 76.8 km, not 64.
- (d)10 km/h — 10 km/h for 4.8 hours covers only 48 km. The rider has done 64 km, so the speed must be higher.
Concept
Speed is distance covered ÷ time taken, and both must describe the same stretch of the ride. The 80 km is the whole route; only 4⁄5 of it, 64 km, was covered in the time given.
Minutes become hours before you divide when the answer is in km/h: 48 minutes is 48⁄60 = 0.8 hour, not 0.48.
13.33 is a rounded value. The exact speed is 40⁄3 = 13 1⁄3 km/h.
Key facts
- Minutes to hours: divide by 60, so 48 min = 0.8 h and 4 h 48 min = 4.8 h.
- 4⁄5 of 80 km = 64 km, the distance that goes with the 4.8 hours.
- 64 ÷ 4.8 = 40⁄3 = 13 1⁄3 km/h, shown in the options as 13.33.
Study next
Common traps
- Writing 4 h 48 min as 4.48 hours, which gives about 14.29 km/h
- Dividing the whole 80 km by the time: 80 ÷ 4.8 ≈ 16.67 km/h
15 Sep 2025, 16:00, Quant Q.18 also turns a clock time into hours first: 1 hour 45 minutes is 1.75 h, so the van's route is 60 × 1.75 = 105 km.
23 Sep 2024, 12:30, Quant Q.9 gives a boat's round trip as 7 hours 30 minutes, which enters the equation as 7.5 h.
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