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Look at Theory of Blackjack, 6th ed. (1999) by Peter Griffin.
There are charts detailing the exact effect of removal for every card, 7's included.
Do the math and see how much of a surplus or deficit of 7's you will need to
change your strategy when your holding is a hard 12, 13, or 14.
Do the (simple) arithmetic and you will learn a lot about BJ.