Important for :

1

C

As per the given information, we get

Average of 15 numbers = 7. So, total of the numbers = 15 × 7 = 105

Average of first 8 numbers = 6.5. So, total of the numbers = 8 × 6.5 = 52

Average of last 8 numbers = 9.5. So, total of the numbers = 8 × 9.5 = 76

Hence, the 8th number = (52 + 76) – 105 = 128 – 105 = 23.

Hence, option C is correct.

Hence, option C is correct.

2

3

4

B

As per the given information, we get

Average of 12 months = 3400. So, total salary of all 12 months = 3400 × 12 = ₹ 40800 (eq. 1)

Average of first 8 months = 3160. So, total salary of first 8 months = 3160 × 8 = ₹ 25280 (eq. 2)

Average of last 5 months = 4120. So, total salary of first 5 months = 4120 × 5 = ₹ 20,600 (eq. 3)

Person's income in the eighth month = (25280 + 20600) – 40800 = 45880 – 40800 = ₹ 5080.

**Note: In such questions, when we calculate total of two different sets (for instance, first 8 months + last 5 months), one particular value (8th month in this case) is calculated twice. **

On subtracting the total of eq. 1 from the total of eq. 2 and 3 we are left with the value of the month that's been calculated twice in the question.

Hence, option B is correct.

On subtracting the total of eq. 1 from the total of eq. 2 and 3 we are left with the value of the month that's been calculated twice in the question.

Hence, option B is correct.

5