Šárka declared:

"I am the youngest of three sisters. Líba is three years older, and Eliška is eight years older. Our mom loves to hear that the average age of all of us (including her) is 21. When I was born, mom was already 29."

How many years ago was Šárka born? 

(M. Volfová)
If we denote Śárka's age in years by $x$, then Líbě is $x+3$, Eliška is $x+8$, and the mother is $x+29$ years old. The average age of all of them is 21 years, i.e.,

$$
(x+(x+3)+(x+8)+(x+29)): 4=21,
$$

after rearrangement

$$
\begin{aligned}
4 x+40 & =84, \\
x & =11 .
\end{aligned}
$$

Śárka was born 11 years ago.