Average

LSSSSB Non-Technical · Mathematics · 56 practice questions with answers and explanations: 18 Easy, 36 Medium, 2 Hard.

Sample questions

Sample Easy question 1

Find the average of all multiples of 5 from 5 to 45.

  1. 25
  2. 23
  3. 27
  4. 22
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Answer: A. This is an arithmetic sequence from 5 to 45; the average of an AP equals the average of its first and last terms: (5+45)/2 = 25.

Sample Easy question 2

The average of two numbers is 35. If one number is 40, find the other.

  1. 30
  2. 34
  3. 32
  4. 28
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Answer: A. Sum = 35 × 2 = 70. Other number = 70 - 40 = 30.

Sample Easy question 3

The average of 20 numbers is 15. If each number is multiplied by 3, find the new average.

  1. 50
  2. 45
  3. 30
  4. 40
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Answer: B. Multiplying every number by 3 multiplies the average by 3 as well: 15 × 3 = 45.

Sample Medium question 4

The average age of 6 members of a committee is 24 years. When one member aged 11 years is replaced by a new member, the average age rises by 3 years. Find the age of the new member.

  1. 32 years
  2. 26 years
  3. 29 years
  4. 14 years
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Answer: C. Total age increases by 6 × 3 = 18 years, so the new member is 11 + 18 = 29 years old.

Sample Medium question 5

The average age of 5 members of a committee is 23 years. When one member aged 11 years is replaced by a new member, the average age rises by 6 years. Find the age of the new member.

  1. 35 years
  2. 47 years
  3. 41 years
  4. 17 years
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Answer: C. Total age increases by 5 × 6 = 30 years, so the new member is 11 + 30 = 41 years old.

Sample Medium question 6

The average age of 9 members of a committee is 34 years. When one member aged 35 years is replaced by a new member, the average age rises by 6 years. Find the age of the new member.

  1. 41 years
  2. 89 years
  3. 95 years
  4. 83 years
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Answer: B. Total age increases by 9 × 6 = 54 years, so the new member is 35 + 54 = 89 years old.

Sample Hard question 7

The average of 12 numbers is 18. If the average of the first 7 is 20 and the average of the last 6 is 15, find the 7th number.

  1. 15
  2. 17
  3. 14
  4. 16
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Answer: B. Total of 12 = 216. First 7 total = 140, last 6 total = 90. 7th number is counted in both groups, so 140 + 90 - 216 = 14 — the 7th number is 14.

Sample Hard question 8

The average of 11 numbers is 50. The average of the first 6 is 49 and of the last 6 is 52. Find the 6th number.

  1. 55
  2. 54
  3. 56
  4. 57
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Answer: C. Sum of all 11 = 550, first 6 sum = 294, last 6 sum = 312. The 6th number is counted in both: 294 + 312 - 550 = 56.

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