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​If two events AAA​ and BBB​ such that P(A∪B)=78P(A∪B)=\frac{7}{8}P(A∪B)=87​​ and P(A∩B)=14P(A∩B)=\frac{1}{4}P(A∩B)=41​​ and P(Aˉ)
Question

If two events AA​ and BB​ such that P(AB)=78P(A∪B)=\frac{7}{8}​ and P(AB)=14P(A∩B)=\frac{1}{4}​ and P(Aˉ)=58P(\bar{A})=\frac{5}{8}​, then P(AˉBˉ)=P(\bar{A}∪\bar{B})=​ ?

A.

14\frac{1}{4}​​

B.

34\frac{3}{4}​​

C.

38\frac{3}{8}​​

D.

18\frac{1}{8}​​

Correct option is B

Given P(A)=58, we know:P(A)=1P(A)=158=38Step 2: Find P(B)Using the formula for the union of two events:P(AB)=P(A)+P(B)P(AB)Substitute the known values:78=38+P(B)14Convert 14 to eighths:78=38+P(B)28Simplify:78=18+P(B)Solve for P(B):P(B)=7818=68=34Step 3: Find P(B)P(B)=1P(B)=134=14Step 4: Compute P(AB)Using De Morgan’s Law:AB=ABThus:P(AB)=P(AB)=1P(AB)=114=34\begin{aligned}&\text{Given } P(\overline{A}) = \frac{5}{8}, \text{ we know:} \\&P(A) = 1 - P(\overline{A}) = 1 - \frac{5}{8} = \frac{3}{8} \\\\&\textbf{Step 2: Find } P(B) \\&\text{Using the formula for the union of two events:} \\&P(A \cup B) = P(A) + P(B) - P(A \cap B) \\&\text{Substitute the known values:} \\&\frac{7}{8} = \frac{3}{8} + P(B) - \frac{1}{4} \\&\text{Convert } \frac{1}{4} \text{ to eighths:} \\&\frac{7}{8} = \frac{3}{8} + P(B) - \frac{2}{8} \\&\text{Simplify:} \\&\frac{7}{8} = \frac{1}{8} + P(B) \\&\text{Solve for } P(B): \\&P(B) = \frac{7}{8} - \frac{1}{8} = \frac{6}{8} = \frac{3}{4} \\\\&\textbf{Step 3: Find } P(\overline{B}) \\&P(\overline{B}) = 1 - P(B) = 1 - \frac{3}{4} = \frac{1}{4} \\\\&\textbf{Step 4: Compute } P(\overline{A} \cup \overline{B}) \\&\text{Using De Morgan's Law:} \\&\overline{A \cup B} = \overline{A} \cap \overline{B} \\&\text{Thus:} \\&P(\overline{A} \cup \overline{B}) = P(\overline{A} \cap \overline{B}) = 1 - P(A \cap B) = 1 - \frac{1}{4} = \frac{3}{4}\end{aligned}​​

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