What preposition should I use in the blank?
Can you change a twenty-dollar ($20) note _____ two ten-dollar ($10) notes?
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The most appropriate preposition is for:
Can you change a twenty-dollar ($20) note for two ten-dollar ($10) notes?
This is the idiomatic way of asking for money to be exchanged.
Some people might use "into", but technically that is wrong. Changing something into something else means to transform it so that it becomes something new. For example, a tadpole changes into a frog.