7.5. In the village of Matitika, along a straight road, five friends live in the following order: Alya, Bella, Valya, Galina, and Dilya. Each of them found the sum of the distances (in meters) from her house to the houses of the others. Bella named the number 700, Valya - 600, Galina - 650. How many meters are there between Bella's and Galina's houses?
Answer: 150 meters.

Solution. Let's denote the houses of the friends with the letters A, B, V, G, D.

It is easy to see that the total distance from B to the other houses is AB + 3BV + 2VG + GD, and from V to the other houses is AB + 2BV + 2VG + GD. These values differ by BV, so the distance between the houses B and V is $700-600=100$ meters.

Similarly, the distance between the houses V and G is $650-600=50$ meters.

Therefore, the distance between the houses B and G is $100+50=150$ meters.

Comment. By similar reasoning, the following result can be obtained: if Bella named the number $x$, Vanya $-y$, Galya $-z$, then the distance between Bella's and Galya's houses is $x+z-2y$ meters.