125. A steamship from Kyiv to Kherson takes three days, and from Kherson to Kyiv - four days (without stops). How long will it take for rafts to float from Kyiv to Kherson?
Solution. To answer the question of the problem, the distance from Kyiv to Kherson needs to be divided by the speed of the Dnieper current. If $s$ is the distance from Kyiv to Kherson, then the speed of the steamboat downstream is $-\frac{s}{3}$ (km/day), and the speed of the steamboat upstream is $-\frac{s}{4}$ (km/day). Then, the speed of the Dnieper current, which is half the difference between the downstream and upstream speeds, is 
$\left(\frac{s}{3}-\frac{s}{4}\right): 2=\frac{s}{24}$ (km/day). Therefore, the required time is $s: \frac{s}{24}=24$ (days).

Answer: 24 days.

$\triangle$ 126. The brother says to the sister: “When Aunt Katya was as old as we are together now, you were as old as I am now. And when Aunt Katya was as old as you are now, you were then ... . How old was the sister then

Answer: the sister was born that year.