Let's calculate the average of the given discrete series using the Short-cut Method with an assumed mean of 25.
### Step-by-Step Solution:
1. List the Sizes and their Frequencies:
[tex]\[
\begin{array}{|c|c|}
\hline
\text{Size} & \text{Frequency} (f) \\
\hline
30 & 2 \\
29 & 4 \\
28 & 5 \\
27 & 3 \\
26 & 2 \\
25 & 7 \\
24 & 1 \\
23 & 4 \\
22 & 5 \\
21 & 7 \\
\hline
\end{array}
\][/tex]
2. Calculate the Deviations from the Assumed Mean (d = x - A):
Given the assumed mean [tex]\( A = 25 \)[/tex],
[tex]\[
\begin{array}{|c|c|}
\hline
\text{Size} & \text{Deviation} (d) \\
\hline
30 & 30 - 25 = 5 \\
29 & 29 - 25 = 4 \\
28 & 28 - 25 = 3 \\
27 & 27 - 25 = 2 \\
26 & 26 - 25 = 1 \\
25 & 25 - 25 = 0 \\
24 & 24 - 25 = -1 \\
23 & 23 - 25 = -2 \\
22 & 22 - 25 = -3 \\
21 & 21 - 25 = -4 \\
\hline
\end{array}
\][/tex]
So the deviations [tex]\( d \)[/tex] are:
[tex]\[
[5, 4, 3, 2, 1, 0, -1, -2, -3, -4]
\][/tex]
3. Compute the Products of Deviations and Frequencies (d \cdot f):
[tex]\[
\begin{array}{|c|c|c|}
\hline
\text{Size} & \text{Deviation} (d) & \text{Product} (d \cdot f) \\
\hline
30 & 5 & 5 \cdot 2 = 10 \\
29 & 4 & 4 \cdot 4 = 16 \\
28 & 3 & 3 \cdot 5 = 15 \\
27 & 2 & 2 \cdot 3 = 6 \\
26 & 1 & 1 \cdot 2 = 2 \\
25 & 0 & 0 \cdot 7 = 0 \\
24 & -1 & -1 \cdot 1 = -1 \\
23 & -2 & -2 \cdot 4 = -8 \\
22 & -3 & -3 \cdot 5 = -15 \\
21 & -4 & -4 \cdot 7 = -28 \\
\hline
\end{array}
\][/tex]
Summing up the products:
[tex]\[
10 + 16 + 15 + 6 + 2 + 0 - 1 - 8 - 15 - 28 = -3
\][/tex]
Sum of [tex]\( d \cdot f \)[/tex] values is [tex]\( -3 \)[/tex].
4. Sum of Frequencies:
[tex]\[
\sum f = 2 + 4 + 5 + 3 + 2 + 7 + 1 + 4 + 5 + 7 = 40
\][/tex]
Total frequency [tex]\( \sum f = 40 \)[/tex].
5. Calculate the Mean Using the Short-cut Method:
[tex]\[
\text{Mean} = A + \frac{\sum (d \cdot f)}{\sum f}
\][/tex]
Plugging in the values:
[tex]\[
\text{Mean} = 25 + \frac{-3}{40}
\][/tex]
[tex]\[
\text{Mean} = 25 - 0.075
\][/tex]
[tex]\[
\text{Mean} = 24.925
\][/tex]
### Final Answer:
The average of the given discrete series, using the Short-cut Method with an assumed mean of 25, is [tex]\( \boxed{24.925} \)[/tex].
