19. A batch of feed can supply 10 ducks and 15 chickens for 6 days, or 12 ducks and 6 chickens for 7 days. Then this batch of feed can supply $\qquad$ ducks for 21 days.
【Solution】Solution: Let 1 duck eat $x$ feed per day, and 1 chicken eat $y$ feed per day. According to the problem, we have:
$$
\begin{aligned}
(10 x+15 y) \times 6 & =(12 x+6 y) \times 7, \\
60 x+90 y & =84 x+42 y \\
24 x & =48 y \\
x & =2 y,
\end{aligned}
$$

Substituting $2 y=x$ into: $(12 x+6 y) \times 7=(12 x+3 x) \times 7=105 x$,
$$
105 x \div 21 x=5 \text { (ducks), }
$$

Answer: This batch of feed can supply 5 ducks for 21 days.
Therefore, the answer is: 5.