The number of loaves that can be made with 2 1/2 bananas is 1 loaf.
The number of loaves that can be made with 5 bananas is 2 loaves.
The amount of bananas needed to make 4 loaves is 10 bananas.
This is a ratio problem, so all you must do is multiply the one number by how much it takes to get to the base ratio with the secondary number.
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