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A person is to count 4500 currency notes...

A person is to count 4500 currency notes. Let `a_(n)` denote the number of notes he counts in the nth minute. If `a_(1) = a_(2) = "………" = a_(10) = 150` and `a_(10) , a_(11),"……….."` are in an A.P. with common difference -2, then the time taken by him to count at notes is `:`

A

34 min

B

125 min

C

135 min

D

24 min

Text Solution

Verified by Experts

The correct Answer is:
A
Till 10th minute, number of counted notes =1500
`:. 3000=(n)/(2){2xx148+(n-1)xx-2}=n(148-n+1)`
`implies n^(2)-149n+3000=0`
`implies (n-125)(n-24)=0`
`:.n=125,24`
`n=125` is not possible.
`:.` n=24`
`:. " Total time =10+24=34 min`.
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