Bunuel wrote:
If the average of the numbers 18, 15, x, 22, 28 and 31 is 21, what is the value of x?
(A) 12
(B) 14
(C) 17
(D) 16
(E) 18
Method 1: Average = Total of the terms ÷ number of terms
=> (18+15+x+22+28+31)/6 = 21
=> 114+x = 126
=> x = 12
Method 2: If we compute the deviations of each of the terms about the average, the SUM of those deviations MUST be ZERO
(This is a very useful concept)
Let us use that:
Deviation of 18 about 21 = 18-21 = -3
Deviation of 15 about 21 = 15-21 = -6
Deviation of 22 about 21 = 22-21 = +1
Deviation of 28 about 21 = 28-21 = +7
Deviation of 31 about 21 = 31-21 = +10
Sum of the above deviations = 9
Hence, deviation of the last number, x, must be -9
Thus, the number x = 21 + (-9) = 12
Answer APosted from my mobile device _________________
Sujoy Kumar Datta |
GMAT Q51 | CAT 99.98 | Mentor & Coach | IIT (Indian Institute of Technology) alumnus
GMAT Prep - personalizedhttps://www.linkedin.com/in/sujoy-kumar-datta/
Sample Videos:
YouTube ChannelReviews: 1. https://cubixprep.com/student-reviews/ | 2. https://cubixprep.com/student-results/
Email: sujoy.datta@gmail.com | Skype: sk_datta | Whats-App: +919433063089 | Tele-gram: @Sujoy_Kr_Datta