A) \[20\]
B) \[25\]
C) \[12\]
D) \[24\]
Correct Answer: C
Solution :
\[\therefore \] \[[10n+2(n-1)10+2(n-2)10+...\] \[+1\times 2\times 10]+[1\times 2\times 10+2\times 2\times 10+...\] \[+2(n-1)\times 10+2n\times 10]\] \[=3000\] (given) \[\Rightarrow \]\[10[4(1+2+3+...+n)-n]=3000\] \[\Rightarrow \] \[4\times \frac{n(n+1)}{2}-n=300\] \[\Rightarrow \] \[2{{n}^{2}}+2n-n=300\] \[\Rightarrow \] \[n=\frac{-1\pm \sqrt{1\pm 2400}}{2\times 2}=\frac{-1\pm 49}{4}\] \[\Rightarrow \] \[n=\frac{48}{4}=12\]You need to login to perform this action.
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