There are three story books and four science and technology books in the bookcase. How many books should you take out at least? How many science and technology books should we take out at least to ensure that we can take out two at a time?
Two books of the same kind are guaranteed, at least:
2x1 + 1 = 3 (this)
How many science and technology books should we take out at least to ensure that we can take out two at a time?
Take at least three story books, so at least:
3 + 2 = 5
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