2. Masha, Dasha, and Sasha are tasked with harvesting all the currants from the bushes on the garden plot. Masha and Dasha together can collect all the berries in 7 hours 30 minutes, Masha and Sasha - in 6 hours, and Dasha and Sasha - in 5 hours. How many hours will it take the children to collect all the berries if they work together?
Solution: Let the entire work be 1. Let $x$ (parts of the entire work per hour) be Masha's labor productivity, $y$ be Dasha's, and $z$ be Sasha's. Since the labor productivity adds up when working together, we can set up a system of equations based on the problem's conditions: $\left\{\begin{array}{l}(x+y) \cdot 7.5=1 \\ (x+z) \cdot 6=1 \\ (y+z) \cdot 5=1\end{array} ;\left\{\begin{array}{l}(x+y)=\frac{2}{15} \\ (x+z)=\frac{1}{6} . \\ (y+z)=\frac{1}{5}\end{array}\right.\right.$. By adding all the equations in the system, we get: $2(x+y+z)=\frac{2}{15}+\frac{1}{6}+\frac{1}{5}=\frac{15}{30}=\frac{1}{2} ; x+y+z=\frac{1}{4}$. Therefore, the three children will gather all the currants together in 4 hours.

Answer: 4.