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Apple = 7 ok
Grape = 12 ok
"3 banana" = 7-1=6
Apple + Grape + "1 banana" NOT solvedable because don't know the value of "1 banana", we only know the value of "3 banana". That's my take!

Well you atually said 3 banana and 1 banana so 1b = 3b/3

My answer is
Apple = 7
Grape = 12
One Banana = 2

therefore 7+12+2 is 21

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