We can use the next formula
[tex]A=P(1+\frac{r}{n})^{nt}[/tex]Where A is the amount, P is the principal, r is the rate n is the periods and t is the time
in our case
A=6000
n=4
r=0.08
t=15 years
We substitute in the formula
[tex]6000=P(1+\frac{0.08}{4})^{4(15)}[/tex]then we isolate the Principal
[tex]P=\frac{6000}{(1+\frac{0.08}{4})^{4(15)}}=1828.70[/tex]The principal is $1828.70
ANSWER
You need to deposit $1828.70 in an account now in order to have $6000
