4. The grandmother and granddaughter were picking strawberries. The granddaughter was using a children's bucket with a capacity of 2.5 liters, while the grandmother was using a two-liter jar. The grandmother had poor eyesight and found it hard to bend down, so the granddaughter was always picking berries faster than her. When the grandmother had filled half of her jar, they switched their containers and, after some time, filled them simultaneously. How many liters of berries did the granddaughter collect in total, assuming that the work efficiency of both the grandmother and the granddaughter remained constant throughout?
Solution. Let the granddaughter collect $X$ liters of berries in the time it takes the grandmother to collect 1 liter, $x>1$ according to the problem. After swapping containers, the granddaughter collected 1 liter of berries, while the grandmother collected $(2.5-x)$ liters. Since their work efficiency does not change, the ratio of the amount of berries collected remains constant.

We can set up the equation: $\frac{1}{x}=\frac{2.5-x}{1} ; 2 x^{2}-5 x+2=0$, which has two roots $x_{1}=2, x_{2}=0.5$. The second root does not fit the problem's conditions. In total, the granddaughter collected $2+1=3$ liters of berries.

Answer: 3.