SSC CHSL 2021 PYQ — Question 72

The correct answer is Option 3 i.e. 47.5.

Formula USed:

Sum of numbers = Average of numbers × Number of terms

Explanation:

According to the given question,

Sum of 18 numbers = 42 × 18 = 756

Sum of last 10 numbers = 40 × 10 = 400

Sum of first 5 numbers = 44 × 5 = 220

Now 220 + 6^th + 7^th + 8^th + 400 = 756

⇒ 6^th + 7^th + 8^th = 756 - 620

⇒ 6^th + 7^th + 8^th = 136    ...(i)

Now, According to th given values, 7^th = 6^th - 6 ⇒ 6^th = 7^th + 6

and 7^th = 8^th - 7 ⇒ 8^th = 7^th + 7

Putting these values in equation(i), we get

⇒ 7^th + 6 + 7^th + 7^th + 7 = 136

⇒ 3(7^th) = 123

⇒ 7^th = 41

Now, 6^th = 41 + 6 = 47

8^th = 41 + 7 = 48

Now, the required average = ${47 + 48} \over 2$ = 47.5

Hence the required answer is 47.5.