A man was employed to count Rs10710. He counts at the rate of Rs180 per minute for half an hour. After this, he counts at the rate of Rs3 less every minute than the preceding minute . Find the time taken by him to count the entire amount.
Amount to be counted =Rs10710
Man counts Rs180perminute for half an hour
Amount counted in 30min=30×Rs180=Rs5400
Amount to be counted =Rs10710–Rs5400=Rs5310
It is also given that after half an hour Rs3 gets less every minute than the preceding minute.
The AP becomes, 177+174+171…+3=5310
Here a=177,d=174–177=−3,Sn=5310
Sum of n terms of AP is =90n
⇒5310=90n∴n=59
Hence the total time taken by the man to count the entire amount =30+59=89minutes or 1hour29minutes