Three classes X, Y and Z take an algebra test.

The average score in class X is 83.

The average score in class Y is 76.

The average score in class Z is 85.

The average score of all students in classes X and Y together is 79.

The average score of all students in classes Y and Z together is 81.

What is the average for all the three classes?

a)
82

b)
81

c)
81.5

d)
84.5

Correct answer is option 'C'. Can you explain this answer?

Question

Three classes X, Y and Z take an algebra test.

The average score in class X is 83.

The average score in class Y is 76.

The average score in class Z is 85.

The average score of all students in classes X and Y together is 79.

The average score of all students in classes Y and Z together is 81.

What is the average for all the three classes?

a)
82
b)
81
c)
81.5
d)
84.5

Correct answer is option 'C'. Can you explain this answer?

Answer

For a second, let us assume the distance is 'x'

So total distance will be '2x'

According to the formula

Avg Speed = Total distance/total time

time for 84km/hr speed = x/84

time for 56km/hr speed = x/56

On solving, you will get 2/5*168 = 67.2

Thanks and Regards

Aviral Srivastava

Answer

Distance from A to B=d1 distance from B to A=d2 time from A to B = t1 time from B to A =t2 speed from A to B=s1 speed from B to A=s2 t1=d1/s1=778/84=9.26 or 9hrs t2=d2/s2=778/56=13.89 or 14hrs average of t1 and t2=(9+14)/2=23/2=11.5 now average speed=778/11.5=67.6 nearest one in answer C 67.2

Answer

Answer

Avg speed of train = 2xy/x+y
⇒ [2(84)(56)]/(84+56)
⇒ [2(84)(56)]/140
= 67.2km/hr

Answer

Avg speed of train = 2xy/x+y
⇒ [2(84)(56)]/(84+56)
⇒ [2(84)(56)]/140
= 67.2km/hr

Answer

Given: Distance between two stations A and B = 778 km, Speed while going from A to B (v1) = 84 km/hr, Speed while returning from B to A (v2) = 56 km/hr

To find: Average speed of the train during the whole journey

Formula to find average speed = Total distance / Total time

Calculation:

Let us assume that the train takes t1 hours to reach from A to B and t2 hours to return from B to A.

Total distance covered by the train = Distance covered while going from A to B + Distance covered while returning from B to A = 778 + 778 = 1556 km

Total time taken by the train = Time taken while going from A to B + Time taken while returning from B to A = t1 + t2

Time taken while going from A to B = Distance / Speed = 778 / 84 = 9.26 hours

Time taken while returning from B to A = Distance / Speed = 778 / 56 = 13.89 hours

Total time taken by the train = t1 + t2 = 9.26 + 13.89 = 23.15 hours

Average speed of the train = Total distance / Total time = 1556 / 23.15 = 67.2 km/hr

Therefore, the average speed of the train during the whole journey is 67.2 km/hr. Hence, option (c) is the correct answer.

Answer

Avg speed of train = 2xy/x+y
⇒ [2(84)(56)]/(84+56)
⇒ [2(84)(56)]/140
= 67.2km/hr

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