7. Increase Grisha's catch by $40 \%$ and Vasya's catch by $20 \%$.

Grisha, the most resourceful of them, calculated that in the first case their total catch would increase by 1 kg; in the second case - decrease by 0.5 kg; in the third case - increase by 4 kg. What was the total catch of the friends (in kilograms) before meeting with Shapka?
Answer: 15.

Solution: Let the harvest of Vasya, Misha, and Grisha be denoted by $x, y, z$ respectively. Then, $0.1 x + 0.2 y = 1; 0.1 y - 0.1 z = -0.5; 0.4 z + 0.2 x = 4$. Adding the equations, we get $0.3(x + y + z) = 4.5$, from which $x + y + z = 15$.