Greg’s solution was 20-11(the starting point-ending point)/14(the number of days).

But the question of “the average daily drop” is a bit confusing because the wind speed either drops or increases over 14 days. Shouldn’t “daily change” be more appropriate word?

Also, when I add up all the daily changes in wind speed and divide the sum(which is -13) by 14(not sure if it should be divided by 13, but either divide 13 or 14, result not the same), the result was not the same as 20-11/14. Can anyone help me with this problem?

We will use **down** for a drop and **up** for the rise:

\begin{aligned}
\textcolor{red}{\text{Down: }}{20 \rightarrow 6}=-14\\
\textcolor{green}{\text{Up: }}{6\rightarrow 8}=+2\\
\textcolor{green}{\text{Up: }}{8\rightarrow 12}=+4\\
\textcolor{green}{\text{Up: }}{12\rightarrow 16}=+4\\
\textcolor{green}{\text{Up: }}{16\rightarrow 20}=+4\\
\textcolor{red}{\text{Down: }}{20 \rightarrow 16}=-4\\
\textcolor{red}{\text{Down: }}{16 \rightarrow 6}=-10\\
\textcolor{green}{\text{Up: }}{6\rightarrow 9}=+3\\
\textcolor{green}{\text{Up: }}{9\rightarrow 13}=+4\\
\textcolor{red}{\text{Down: }}{13 \rightarrow 5}=-8\\
\textcolor{green}{\text{Up: }}{5\rightarrow 6}=+1\\
\textcolor{green}{\text{Up: }}{6\rightarrow 8}=+2\\
\textcolor{green}{\text{Up: }}{8\rightarrow 11}=+3\\
\end{aligned}

\dfrac{-14+2+4+4+4-4-10+3+4-8+1+2+3}{14}= |\dfrac{-9}{14}|=|-0.64|=0.64 \text{ or } \dfrac{-9}{13}=0.69

Oh, thanks for your kind explanation. But isn’t it tricky to accurately measure the exact value of each change? I mean, there are four parts where you and I saw the number differently. On Day 3, I read the scale at 7 where you read 8. On day 5 and day 7, I read the scale at 17 whereas you read it at 16. On day 8, I read the scale at 5 whereas you read it at 6. Since the exact scale was not specified in the question, calculation can vary. How can I deal with this issue then?

As you can it’s written **closest** value thus, even if every of your chosen value is \pm2 then also your close approximation should be in the vicinity of 0.65. The question is more about the trick \dfrac{\text{Range}}{\text{Total Number of Value}} and if you don’t know the trick then choose the **Closest Values**

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