There are **55 weeks** in 18 months.

Here's the calculation:

- Since a year has 12 months, 18 months is equivalent to 18 months / 12 months/year = 1.5 years.

- We know there are 52 weeks in a year.

- Therefore, the number of weeks in 1.5 years is 1.5 years * 52 weeks/year = 78 weeks.

However, this calculation doesn't account for the remaining months beyond the full years. Since a month has approximately 4.345 weeks, we can estimate the remaining weeks by multiplying the remaining months by the average number of weeks per month: (0.5 years * 12 months/year) * 4.345 weeks/month ≈ 26 weeks.

Adding the full years' weeks and the estimated remaining weeks, we get 78 weeks + 26 weeks ≈ 55 weeks.

Here's the calculation:

- Since a year has 12 months, 18 months is equivalent to 18 months / 12 months/year = 1.5 years.

- We know there are 52 weeks in a year.

- Therefore, the number of weeks in 1.5 years is 1.5 years * 52 weeks/year = 78 weeks.

However, this calculation doesn't account for the remaining months beyond the full years. Since a month has approximately 4.345 weeks, we can estimate the remaining weeks by multiplying the remaining months by the average number of weeks per month: (0.5 years * 12 months/year) * 4.345 weeks/month ≈ 26 weeks.

Adding the full years' weeks and the estimated remaining weeks, we get 78 weeks + 26 weeks ≈ 55 weeks.