Find the digits that are missing in each of the spaces marked with dashes.
(a) $\frac{126}{8_{-}}=\frac{21}{--}$
(b) $\frac{--8}{33 \_}=\frac{4}{5}$
(a) Observe that $21 \times 6=126$. Therefore, the numerator 21 was multiplied by 6 to obtain the numerator 126 of the first quotient. Thus, the denominator _- was also multiplied by 6 to give the denominator $8_{-}$ of the first quotient. The only number of the form 8_ that is divisible by 6 is 84, and $84 \div 6=18$. We can then complete the fractions, obtaining

$$
\frac{126}{8 \underline{4}}=\frac{21 \times 6}{18 \times 6}=\frac{21}{1 \underline{8}}
$$

(b) We have $4 / 5=0.8$ and we want to have _- $8=0.8 \times 33_{-}$. Notice that 33_ must be a multiple of 5, so it can only be 330 or 335. However,

$$
0.8 \times 330=264 \neq \_\_8
$$

Thus, $33_{-}=335$, with $335=5 \times 67$, and $_{-\_} 8=268=4 \times 67$. Therefore, there is only one way to complete the fraction,

$$
\frac{2 \underline{6} 8}{33 \underline{5}}=\frac{268 \div 67}{335 \div 67}=\frac{4}{5}
$$