Is this for Math Challenge II-A Winter? Please remember to always include the name of the course you are asking about. Also, try to include what have you tried so far so we can give you better hints.

Here are some ideas you can use for 24: list them; use divisibility rule for 9; use the fact that the multiples look like $9 \times 2$, $9 \times 3$, etcetera; there are $\left\lfloor\dfrac{100}{9}\right\rfloor = 11$ multiples to $9$ between $1$ and $100$.

For 30, as Charles mentioned, use complementary counting.