Is it correct to ask somebody,how manyth son are you to your father?
he is the sixth son of david. then how can we ask it as a question.

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If you want to know how many sons someone's father has in english you would ask him :
:"How many brothers do you have ?" This is the correct way to know how many sons his father has , but the other expression is not correct in english !

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It's not correct to ask that; "Manyth" is not actually an english word.

You could ask "What son are you? The first, the second...", or "What number son are you?"

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What is your birth order? Does this sounds good!?