Unemployment and Labor Force MacroEconomics
A short note: please try to show your math work in the original post, and maybe learn some basic Math TeX commands. It makes people much more willing to help with homework questions.

So first, we have our unemployment for the year 2000. Let $U_t, u_t, L_t$ denote the unemployment rate, number of unemployed, and number of laborers at time $t$, respectively.

$$U_t = \frac {u_t}{u_t + L_t}$$
$$U_{2000} = \frac {2,000,000}{2,000,000 + 16,000,000} = \frac{1}{9} \approx 0.111$$

So in the year 2001, 800,000 people turn 16, but half do not enter the labor force. So the question is how many people have to find a job for unemployment to fall below 8%.

$$U_{2001} = \frac{2,000,000 + (400,000 - n)}{2,000,000 + (400,000 - n) + 16,000,000 + n} = 0.08$$

where $n$ is the number of those who found jobs, and $400,000 - n$ is the net change in unemployment numbers. Solve for $n$.

$$\frac{2,400,000 - n}{18,400,000} = 0.08$$
$$\implies 1,472,000 = 2,400,000 - n \implies 928,000 = n$$

So what does this tell us? Even if all 400,000 new 16 year olds found jobs, that would not drive unemployment down to 8%. You need quite a few more people to find jobs.