4. Gill went for a five-hour walk. Her average speed was between $3 \mathrm{~km} / \mathrm{h}$ and $4 \mathrm{~km} / \mathrm{h}$. Which of the following could be the distance she walked?
A $12 \mathrm{~km}$
B $\quad 14 \mathrm{~km}$
C $19 \mathrm{~km}$
D $24 \mathrm{~km}$
E $35 \mathrm{~km}$
Solution: $\mathbf{C}$
Walking at $3 \mathrm{~km} / \mathrm{h}$ for five hours, Gill would walk $5 \times 3 \mathrm{~km}=15 \mathrm{~km}$, and at $4 \mathrm{~km} / \mathrm{h}$ she would walk $5 \times 4 \mathrm{~km}=20 \mathrm{~km}$. As her average speed is between $3 \mathrm{~km} / \mathrm{h}$ and $4 \mathrm{~km} / \mathrm{h}$, she walks between $15 \mathrm{~km}$ and $20 \mathrm{~km}$. Of the options given, only $19 \mathrm{~km}$ is in this range.