How many months are there in 15 weeks?

To be exact would require knowledge of when the 15 weeks were due to the different number of days in each month; however an approximate calculation can be done using an average month of 52 weeks ÷ 12 months = 41/3 weeks per month:

41/3 weeks ≈ 1 month

→ 15 weeks ≈ 15 ÷ 41/3 months

= 15 ÷ 13/3 months

= 15 x 3/13 months

= 36/13 months

≈ 31/2 months (a little under)