Let the number of boxes filled, by using 20 bottles in each box, be x.

The following table is obtained.

Number of bottles | 12 | 20 |

Number of boxes | 25 | x |

More the number of bottles, lesser will be the number of boxes.

Hence, the number of bottles and the number of boxes required to pack these are

inversely proportional to each other.

Therefore,

12 × 25 = 20× x

x = (12 x 25)/20 = 15

Hence, the number of boxes required to pack these bottles is 15.