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