In order to determine how much is the amount the woman should invest, use the following formula for the amount of money earnt with compound interest:

where,
A: amount obtained after t years = $45,000
P: principal investment = ?
r: percentage rate = 6% = 0.036
n: times at year = 2 (semiannually)
t: time = 15 years
Replace the previous values of the parameters and then solve for P, as follow:

Hence, the woman should invest approximately $18,539.40