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A man runs a distance of 9km at a constant speed for the first 4km and then 2km/hr faster for the rest of the distance. The whole run takes him 1 hour. His average speed for the first 4km is

A. 6km/hr

B. 9km/hr

C. 8km/hr

D. 11km/hr

QUICK ANSWER…

C

DETAILS…

Let the average speed for the first 4km be = X

Total distance = 9km

Speed for the rest of the distance (which is 5km) = X + 2

So,

But recall that distance = speed × time

Total distance = total average speed × total time

Total distance = (X + 1) × 1 = 9

So, X + 1 = 9

X = 9 – 1 = 8km/hr

Now for the right answer to the above question:

1. Option A is incorrect.
2. Option B is incorrect.
3. C is correct.
4. D is not the correct answer.

KEY-POINTS…

You may please note these/this:

• There’s another method for resolving this, but we did not feature it here because it lead to simultaneous equations which are somehow longer to solve.
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/ culled from 2020 JAMB-UTME mathematics past question 01 /