Discussion
The average attendance in a school for the first 4 days of the week is 30 and for the first 5 days of the week is 32. The attendance on the fifth day is
Answer: B
The attendance on the fifth day is 40.
Convert each average into a total:
\[ \begin{aligned} \text{first 4 days} &= 4 \times 30 = 120 \\ \text{first 5 days} &= 5 \times 32 = 160 \\ \text{fifth day} &= 160 - 120 = 40 \end{aligned} \]
Shortcut: the fifth day must lift the average of five days by 2, so it is the old average plus \(5 \times 2\): \(30 + 10 = 40\). 32 is the new average itself, a common slip, and 36 and 38 do not give a five-day total of 160.
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