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Mathematics · 2025 · 4 marks
CBSE 2025 · Region 6 · Set 1 · Q38
A shop selling electronic items sells smartphones of only three reputed companies A, B and C because chances of their manufacturing a defective…
A shop selling electronic items sells smartphones of only three reputed companies $\displaystyle \mathrm{A}, \mathrm{B}$ and C because chances of their manufacturing a defective smartphone are only $\displaystyle 5 \%, 4 \%$ and $\displaystyle 2 \%$ respectively. In his inventory he has $\displaystyle 25 \%$ smartphones from company A, $\displaystyle 35 \%$ smartphones from company B and $\displaystyle 40 \%$ smartphones from company C . A person buys a smartphone from this shop.
(i)
Find the probability that it was defective.
(ii)
What is the probability that this defective smartphone was manufactured by company B ?
Marking-scheme solution
(i)
$\displaystyle \mathrm{P}($ defective smartphone $\displaystyle )=0.25 \times 0.05+0.35 \times 0.04+0.40 \times 0.02$
\[=0.0345
\]
(ii)
$\displaystyle \mathrm{P}(\mathrm{B} /$ Defective $\displaystyle )=\frac{0.35 \times 0.04}{0.25 \times 0.05+0.35 \times 0.04+0.40 \times 0.02}$
\[=\frac{140}{345} \text { or } \frac{28}{69}
\]
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CBSE Class 12 Mathematics past-paper question from the 2025 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.