Question 980568
Let x = P(B)



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



1/9 = 1/7 + x - 1/9



1/9 <font color="blue">+ 1/9</font> = 1/7 + x - 1/9 <font color="blue">+ 1/9</font> ... Add 1/9 to both sides



2/9 = 1/7 + x



x + 1/7 = 2/9



x + 1/7 <font color="blue">- 1/7</font> = 2/9 <font color="blue">- 1/7</font> ... Subtract 1/7 from both sides.



x = 2/9 - 1/7



x = 14/63 - 9/63



x = 5/63



So, P(B) = 5/63