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The answer is
You just need to put the pieces together: you are given the number (although expressed through unknowns) of two subsets that, combined together, compose the whole set you're interested in.
So, if you split all the children between boys and girls, and you know how many boys and girls are at the camp, then you just need to put the boys and girls together again to get the whole number.
And numerically, "putting numbers together" means to add them, hence the answer.