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Let $\mathrm{S}_{\mathrm{n}}=\sum_{\mathrm{k}=1}^{\mathrm{n}} \mathrm{k}$ denote the sum of the first $\mathrm{n}$ positive integers. The numbers $\mathrm{S}_{1}, \mathrm{~S}_{2}, \mathrm{~S}_{3}, \ldots \mathrm{S}_{99}$ are written on 99 cards. The probability of drawing a card with an even number written on it is -
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$49 / 99$
$1,3,6,10,15,21,28,36,45,55,66,78,91,105$ till 98 terms 48 terms are even and 48 terms odd
$99^{\text {th }}$ term $=\frac{99 \times 100}{2}=$ even
Total even terms $=48+1=49$
Probability $=\frac{49}{99}$
$99^{\text {th }}$ term $=\frac{99 \times 100}{2}=$ even
Total even terms $=48+1=49$
Probability $=\frac{49}{99}$
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