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Mathematics

Assertion (A) : The mean of 19 numbers is 38. If the mean of the first 10 numbers is 36 and that of the last 10 is 40, then the 10th number is 38.

Reason (R) : Mean = Sum of observationsNumber of observations\dfrac{\text{Sum of observations}}{\text{Number of observations}}

  1. A is true, R is false

  2. A is false, R is true

  3. Both A and R are true

  4. Both A and R are false

Statistics

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Answer

Given,

Mean of 19 number = 38

⇒ Total sum = 19 × 38 = 722

Mean of first 10 numbers = 36

⇒ Total sum = 10 × 36 = 360

Mean of last 10 numbers = 40

⇒ Total sum = 10 × 40 = 400

The 10th number is included in both groups:

⇒ First 10 numbers → includes 10th

⇒ Last 10 numbers → also includes 10th

So,

⇒ 10th number = (Sum of first 10 numbers + Sum of last 10 numbers)- Sum of all the 19 numbers

= (360 + 400) - 722

= 760 - 722 = 38.

∴ Assertion (A) is true.

We know that,

Mean = Sum of observationsNumber of observations\dfrac{\text{Sum of observations}}{\text{Number of observations}}

∴ Reason (R) is true.

Both Assertion (A) and Reason (R) are true.

Hence, option 3 is the correct option.

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