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Where can we find some information to have our theoretical background in our study? So ive been studying blood types and I m O+ but my daddy is ab- and my mom is a+ how is it possible that I got my blood type? How many different 4-digit numbers can be formed using the digits 2, 4, 5, 6, and 7 so that no digits repeat & the number is divisible by 4? My cursor won't move and the computer is locked. What do I do? What are some ways to demonstrate the density property of the real numbers?