Let the duration of each **, when there are 9 periods a day in the school, be x minutes. The following table is obtained.
| Duration of each ** (in minutes) | 45 | x |
| Number of periods | 8 | 9 |
If there is more number of periods a day in the school, then the duration of each ** will be lesser. Hence, this is a case of inverse proportion.
Therefore 45 × 8 = x × 9
x = (45 x 8)/9 = 40
Hence, in this case, the duration of each ** will be 40 minutes.