A bus covers 330 km in 6 hours. It travels part of the way at 40 km/h and the rest at 60 km/h. How many km does it travel at 40 km/h?

60 kmCorrect answerYour answer82.5 kmCorrect answerYour answer90 kmCorrect answerYour answer110 kmCorrect answerYour answer

Show the answer and explanation

CorrectNot quiteAnswer A

Correct answer: A

Average speed = $\frac{330}{6} = 55$ km/h.
Time at 40 : time at $60 = (60 - 55) : (55 - 40) = 5 : 15 = 1 : 3$ .
Time at 40 km/h = $\frac{1}{4} \times 6 = 1.5$ hours.
Distance = $40 \times 1.5 = 60$ km.

Worth noting

With speeds, alligation gives the ratio of TIMES, because average speed is total distance ÷ total time. Applying $1 : 3$ to the distance gives the wrong 82.5 km; 90 km uses 60 km/h for the 1.5 hours.

Concept: Alligation on speeds gives the ratio of times
Typical timeTypical: 110 s
Your timeYours: —

The rule this testsThe alligation rule

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Ten questions from Mixtures & Alligation, at your own pace.

A bus covers 330 km in 6 hours. It travels part of the way at 40 km/h and the rest at 60 km/h. How many km does it travel at 40 km/h?

Explanation