Answer:
[tex]1998 + 5\sqrt{30}[/tex]
Step-by-step explanation:
Set up the equation:
Since C(t) gives the number of cars purchased in the t-th year after 1998, then make the number of cars equal to 15 000 and solve for t - the year:
20t^2 = 15000
t^2 = 750
t = [tex]\sqrt{750} = \sqrt{10} \cdot \sqrt{25} \cdot \sqrt{3} = 5\sqrt{30}[/tex]
The year will be simply 1998 + 5 \sqrt{30}