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Assuming the left & right shoes have the same value:

Shoes(LR) + Shoes(LR) + Shoes(LR) = 30
Shoes(LR) = 10 --> Shoes (L or R) = 5

Boy + Boy + Shoes(LR) = 20
2 Boy + 10 = 20
Boy = 5

2 Food + 2 Food + Boy = 13
4 Food + 5 = 13
Food = 2

Shoe (L) + Boy x Food = ?
5 + 5x2 = ?
5 + 10 = 15

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