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Context: Equation 1 requires substituting A for B.

Does this mean all "A" in equation 1 is replaced with "B" or vice versa?

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    Vice-versa. You would be replacing all of the Bs with As. – David M Mar 24 '14 at 20:40
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Substituting A for B means that B is replaced by A wherever it occurs.

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It's probably a matter of looking at the original equation. Because the 'for' could be interpreted differently by different folks.

I would take "substituting A for B" to mean replacing B's with A's. It would be more clear if written as "substituting A's with B's."

In the end, you're going to have two equations, one with A's and one with B's. So, whatever you have on hand now, the sentence means create the other.

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    You made a minor error. It would be more clear if written substituting B's with A's. Otherwise, your interpretation is correct. – David M Mar 24 '14 at 20:59
  • Er, yes. The key part is using "with" instead of "for" with the variables in whatever order is desired. – SamC Mar 25 '14 at 14:06
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Whilst I agree with SamC's answer, subject to David M's comment, I also agree, contrary to other comments, that 'substituting A for B' is entirely ambiguous.

'For', in this context, could mean both 'in place of', as well as 'in favour of'.

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