$$First$-$class postage is

$ $0\mathrm{.}38$

for the first ounce$($or any fraction$$ thereof$)$ and

$ $0\mathrm{.}21$

for each additional ounce$($or fraction$$ thereof$).$ Let$$ C$($x$)$ represent the postage for a letter weighing x oz$.$

Use this information to answer the questions.