Q.

A person is to count 4500 currency notes. Let an  denote the number of notes he counts in the nth minute. If a1=a2=...=a10=150  and a10,a11,... are in an AP with common difference -2, then the time taken by him to count all notes is

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a

135 minutes

b

24 minutes

c

34 minutes

d

125 minutes

answer is C.

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Detailed Solution

Till 10th  minute number of counted notes = 1500 3000=n2[2×148+(n−1)(−2)]=n[148−n+1] ⇒n2−149n+3000=0 ⇒n=125,24 ⇒n=125  is not possible⇒  Total time =24+10=34  min
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