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When the confidence level is $95\%$ and the population mean is $10$, the critical value $t*$ is calculated.

Utilize the formula $df=n-1$to estimate the degree of freedom. Where, the population mean is$n=10$.
$df=n-1$
$=10-1=9$
The row in table $B$is represented by the degree of freedom $df$.

Convert the confidence level $95\%$ into decimal.
$\frac{95}{100}=0.95$
Determine the column:
$$\begin{gathered} \frac{1-c}{2}=\frac{1-0.95}{2} \\ =0.025 \end{gathered}$$
Using the table$B$, to determine the critical value $t*,$for row $9$and the column$0.025$:
$t^{*}=2.262$

Therefore, the critical value is $t^{*}=2.262$.

When the confidence level is $99\%$and the population mean is $20$, the critical value $t$ is calculated.

Using the formula $df=n-1$, to determine the degree of freedom. Where, the population mean is$n=20$.
$df=n-1=20-1=19$.

Convert the confidence level $99\%$into decimal as:

$\frac{99}{100}=0.99$

Determine the column by:

$$\begin{gathered} \frac{1-c}{2}=\frac{1-0.99}{2} \\ =0.005 \end{gathered}$$

Using the table B, determine the critical value $t^{*}$, for row $19$and the column $0.005$:

$t^{*}=2.861$

Therefore, the critical value is $t^{*}=2.861$.