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One third of a certain journey is covered at the rate of 25 km/hour, one-fourth at the rate of 30 km/hour and the rest at 50 km/hour. The average speed for the whole journey is
35 km/hour
33
1
km/hr
3
30 km/hour
37
1
km/hr
12
Correct Option: B
Let the total distance be x km.
Total time =
x
x
5x
3
+
4
+
12
25 30 50
=
x
+
x
+
x
75 120 120
=
x
+
x
75 60
=
4x + 5x
=
3x
hours
300 100
∴ Average speed =
Total distance
Time taken
=
x
3x
100
=
100
= 33
1
kmph
3 3
Second Method :
Here, x = 3, u = 25, y = 4, v = 30
z =
12
, w = 50
5
Average Speed =
1
1
+
1
+
1
xu yv zw
=
1
1
+
1
+
1
3 × 25 4 × 30 (12/5) × 50
=
1
1
+
1
+
1
75 120 120
=
1
1
+
1
75 60
=
1
4 + 5
300
=
300
=
100
9 3
= 33
1
km/hr.
3
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