bread = 2
salad = 2
meat = 5
tomatoes = 4

b = bread
t = tomatoes
s = salad
m = meat

2b + t + s = 10
b + s + m = 9 || * (-1)
b + t + s = 8

2b + t + s = 10
-b - s - m = -9
b + t + s = 8

2b + t + s = 10
-m + t = -1     || * (-1)

2b + t + s = 10
m - t = 1

2b + s + m = 11