Average speeds

Level 2

Andy travelled from town A to C via town B, which was at the midpoint between town A and C. His average speed from town A to B and town B to C were 35km/h and 63km/h, respectively. What was his average speed from town A to C?

45km/h 14km/h 98km/h 49km/h

Poh Seng Tan by Poh Seng Tan , 42, Singapore

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1 solution

Poh Seng Tan
May 2, 2018

Let: Distance of A to B = D, which is also the distance from B to C. Time taken from A to B = p Time taken from B to C = q

Since speed from A to B = 35km/h and from B to C = 63km/h

D = 35km/h * p = 63km/h * q

q = 35 * p / 63

Total time from A to C = p + q

Average speed from A to C

= 2 D p + q \frac{2D}{p+q}

= 2 ∗ 35 p p + q \frac{2*35p}{p+q}

= 70 p ( p + 35 p ) / 63 \frac{70p}{(p+35p)/63}

= 70 p ( 63 p + 35 p ) / 63 \frac{70p}{(63p+35p)/63}

= 70 p 98 p / 63 \frac{70p}{98p/63}

= 70 p 14 p / 9 \frac{70p}{14p/9}

= 630 14 \frac{630}{14}

= 45 km/h

0 Helpful 0 Interesting 0 Brilliant 0 Confused

Based on your solution, you clearly meant to include some information about distances in this question; without that any answers between 35 km/h and 63 km/h are possible.

zico quintina - 3 years, 1 month ago

oops, you are right. added in the missing info. Question and answer actually written by my 8 yo son.

Poh Seng Tan - 3 years, 1 month ago

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