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Previous Year Question (PYQs)
1
Consider the sample space $\Omega={\{(x,y):x,y\in{\{1,2,3,4\}\}}}$ where each outcome is equally likely.
Let A = {x ≥ 2} and B = {y > x} be two events. Then which of the following is NOT true?
Solution
We have the sample space
\[
\Omega = \{(x,y) : x,y \in \{1,2,3,4\}\}, \qquad |\Omega| = 16.
\]
Event
\[
A = \{x \ge 2\}.
\]
Values of \(x = 2,3,4\), so total favorable outcomes:
\[
12 \quad \Rightarrow \quad P(A) = \frac{12}{16} = \frac{3}{4}.
\]
Event
\[
B = \{y > x\}.
\]
Count pairs:
\[
\begin{aligned}
x=1 &: (1,2),(1,3),(1,4) \Rightarrow 3, \\
x=2 &: (2,3),(2,4) \Rightarrow 2, \\
x=3 &: (3,4) \Rightarrow 1.
\end{aligned}
\]
Thus total = 6, so
\[
P(B) = \frac{6}{16} = \frac{3}{8}.
\]
Now compute \(A \cap B\):
\[
x \ge 2,\quad y > x.
\]
Valid pairs:
\[
(2,3),(2,4),(3,4).
\]
So
\[
P(A \cap B) = \frac{3}{16}.
\]
Check independence:
\[
P(A)P(B) = \frac{3}{4} \cdot \frac{3}{8} = \frac{9}{32},
\]
but
\[
P(A \cap B) = \frac{3}{16} = \frac{6}{32}.
\]
Since
\[
\frac{9}{32} \neq \frac{6}{32},
\]
events \(A\) and \(B\) are not independent.
Therefore, the NOT true statement is:
\[
\boxed{P(A \cap B) = \frac14}.
\]
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