A man travels equal distances of his journey at 15, 20 and 30 km/hour respectively. What is his average speed for whole journey?

Required average speed
= (3 × 15 × 20 × 30)/(15 × 20 + 20 × 30 + 30 × 15)
= 20 km/hour
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Alternative way:
Distance 30 km, Speed 30 km/h, Time 1 Hour
Distance 30 km, Speed 15 km/h, Time 2 Hours
Distance 30 km, Speed 20 km/h, Time 1 hour & 30 minutes.
Total Distance 90 km, Total Time 4.5 Hours,
Average Speed 90/4.5 km/h = 20 km/h
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Alternative way:
Let, equal distances travelled by the man be ’s’.
Time taken to travel first distance, s at 15 kmph = s/15
Time taken to travel second distance, s at 20 kmph = s/20
Tine taken to travel last distance, s at 30 kmph = s/30
Therefore, total time taken by the man = s(1/15 +1/20 +1/30)
Total distance travelled by the man = 3s
Hence, average speed of the man
= 3s / [ s(1/15 +1/20 +1/30) ] =3/(1/15 +1/20 +1/30)
= 20 kmph
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Alternative way:
Avg speed formula= 3 ÷ (1/x +1/y +1/z)
Now we have 1/30 +1/20 +1/15
L.C.M. of 20, 30 and15 is 60.
Then (3 + 4 + 8)/60=9/60 = 3/20
From above formula, average speed = 3 ÷ 3/20=20 km/h