The interest rate compounded semiannually is 15/2 = 7.5%.
The number of compounding periods is 6 years * 2 = 12.
The formula for compound interest is A = P(1 + r/n)^nt, where:
* A is the final amount
* P is the principal amount
* r is the interest rate
* n is the number of compounding periods per year
* t is the number of years
In this case, we have A = $1800, r = 7.5%, n = 12, and t = 6.
Solving for P, we get:
```
P = A / (1 + r/n)^nt
P = $1800 / (1 + 0.075)^12 * 6
P = $683.27
```
Therefore, you need to deposit $683.27 now to have $1800 in 6 years from now.