4. Yumi has a flat circular chocolate chip cookie with radius $3 \mathrm{~cm}$. On the top of the cookie, there are $k$ circular chocolate chips, each with radius $0.3 \mathrm{~cm}$. No two chocolate chips overlap and no chocolate chip hangs over the edge of the cookie. For what value of $k$ is exactly $\frac{1}{4}$ of the area of the top of the cookie covered with chocolate chips?
4. Since the top of the cookie is a circle with radius $3 \mathrm{~cm}$, its area is $\pi(3 \mathrm{~cm})^{2}=9 \pi \mathrm{cm}^{2}$.
Since a chocolate chip has radius $0.3 \mathrm{~cm}$, its area is $\pi(0.3 \mathrm{~cm})^{2}=0.09 \pi \mathrm{cm}^{2}$.
We are told that $k$ chocolate chips cover $\frac{1}{4}$ of the area of the top of the cookie. Therefore, $k \times 0.09 \pi \mathrm{cm}^{2}=\frac{1}{4} \times 9 \pi \mathrm{cm}^{2}$ and so $0.09 k=\frac{1}{4} \times 9=2.25$ or $k=\frac{2.25}{0.09}=25$.
ANSWER: $k=25$