The ball on the thread makes 26 vibrations in a certain time. The length of the thread was changed by 4 cm

The ball on the thread makes 26 vibrations in a certain time. The length of the thread was changed by 4 cm, the ball will make 24 vibrations. How long is the thread?

Let us express the periods of oscillations in terms of time t, which was the same in the first and second cases:
T1 = t / 26; T2 = t / 24;
Now we write down the time t through periods of oscillations:
t = 26 * T1; t = 24 * T2;
By condition 24 * T1 = 26 * T2 (1).
Answer: 0.23 m (23 cm)