Why did Trump agree to this?

Mexico agreed to put 10,000 of their soldiers on the border to help deal with illegal immigration. This is the total number. Previously they had 15,000 on the border. How is it that they reduced the number by 5,000 and Trump considers it a good thing? The only conclusion I can come to is that he had no idea how many soldiers were already there.

Why did Trump agree to this?
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