Teaser 2611: Simple arithmetic

From The Sunday Times, 7th October 2012 [link] [link]

George and Martha are teaching their great-grandchildren some simple arithmetic. “If you add two thirties to four tens you get a hundred”, George was saying, and he wrote it like this:

“Strangely”, added Martha, there are nine different letters there, and if you allow each letter to stand for a different digit, there is a unique sum which works”.

Which digit would be missing?

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