The number of letters posted in a certain city on each day for a week is given as follows: 40, 55, 28, 25, 31, 52, 43. Which of the following is not an entry in the three-day moving averages?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 37
A three-day moving average is the mean of three consecutive values; the window then moves forward by one day. Seven values give five averages:
\( \displaystyle \frac{40 + 55 + 28}{3} = \frac{123}{3} = 41 \)
\( \displaystyle \frac{55 + 28 + 25}{3} = \frac{108}{3} = 36 \)
\( \displaystyle \frac{28 + 25 + 31}{3} = \frac{84}{3} = 28 \)
\( \displaystyle \frac{25 + 31 + 52}{3} = \frac{108}{3} = 36 \)
\( \displaystyle \frac{31 + 52 + 43}{3} = \frac{126}{3} = 42 \)
The entries are 41, 36, 28, 36 and 42. Of the options, only 37 is not among them.