Step 1: Let the cost price of each article be \( C \).
Assume the total number of articles is 1 (for simplicity).
Step 2: Articles sold at a gain of 20\%
Three-fourths of the articles are sold at a gain of 20\%.
The selling price of each article in this group is \( C \times 1.20 \).
So, the total selling price for three-fourths of the articles is:
\[
\text{Selling price of } \frac{3}{4} \text{ of the articles} = \frac{3}{4} \times C \times 1.20 = 0.90C
\]
Step 3: Articles sold at cost price
The remaining one-fourth of the articles are sold at cost price, so the total selling price of this group is:
\[
\text{Selling price of } \frac{1}{4} \text{ of the articles} = \frac{1}{4} \times C = 0.25C
\]
Step 4: Total selling price
The total selling price for all articles is:
\[
\text{Total selling price} = 0.90C + 0.25C = 1.15C
\]
Step 5: Total cost price
The total cost price of all the articles is:
\[
\text{Total cost price} = C
\]
Step 6: Total gain
The total gain is the difference between the total selling price and the total cost price:
\[
\text{Total gain} = 1.15C - C = 0.15C
\]
Step 7: Percentage gain
The percentage gain is calculated as:
\[
\text{Percentage gain} = \frac{0.15C}{C} \times 100 = 16\%
\]
Thus, the gain earned by the dealer is \( 16\% \).