Posted by **Jim** on Sunday, October 14, 2012 at 2:14pm.

Could someone help be understand the general addition rule for probability? It says P(A or B)= P(A)+P(B) - P(A and B). But I get a different answer when I work it out myself:

P(A or B)= P(A)(1-P(B)) +P(B)(1-P(A))

= P(A) -P(A)P(B) +P(B)-P(A)P(B)

=P(A) +P(B) -2P(A)P(B)

Would someone please help me! Thanks :)

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