Given that we have 25 boxes with 12 bottles in each box and let x be boxes be filled if 20 bottles are in each box.
We can observe that if the number of bottles is increased then the required number of boxes will be decreased. Thus the two quantities are in inverse proportion.
\(\Rightarrow x_1y_1 = x_2y_2\)
\(25 \times 12 = x \times 20\)
\(x = 15\)
Hence, the required number of boxes = 15.