Q. 323.5( 31 Votes )

150 workers were engaged to finish a job in a certain number of days. 4 workers dropped out on second day, 4 more workers dropped out on third day and so on. It took 8 more days to finish the work. Find the number of days in which the work was completed.

Answer :

Let total work = 1


and let total work be completed in 'n' days


Work done in 1 day = 1/n


This is the work done by 150 workers


Work done by 1 worker in one day = 1/150n


Case 1: -


No. of workers = 150


Work done per worker in 1 day = 1/150n


Total work done in 1 day = 150/150n


Case 2: -


No. of workers = 146


Work done per worker in 1 day = 1/150n


Total work done in 1 day = 146/150n


Case 3: -


No. of workers = 142


Work done per worker in 1 day = 1/150n


Total work done in 1 day = 142/150n


Given that


In this manner it took 8 more days to finish the work i.e. work finished in (n + 8) days.




…(1)


Now,


is an AP


where,


first term(a) = 150


common difference(d) = 146 - 150 = - 4


we know that


Sum of n terms of AP(Sn) = (n/2)[2a + (n - 1)d]


putting n = n + 8, a = 150 & d = - 4


Sn + 8 = [(n + 8)/2] × [2(150) + (n + 8 - 1)( - 4)]


= [(n + 8)/2] × [300 + (n + 7)( - 4)]


= [(n + 8)/2] × [300 - 4n - 28]


= [(n + 8)/2] × [272 - 4n]


= (n + 8) × (136 - 2n)


= - 2n2 + 120n + 1088


From (1),


Sn + 8 = 150n


- 2n2 + 120n + 1088 = 150n


- 2n2 - 30n + 1088 = 0


- 2(n2 + 15n - 544) = 0


(n2 + 15n - 544) = 0


n2 + 32n - 17n - 544 = 0


n(n + 32) - 17(n + 32) = 0


(n - 17)(n + 32) = 0


n = 17


because n = - 32 is invalid as no. of days can't be - ve.


Hence, n =17


Thus, the work was completed in n + 8 days i.e. 17 + 8 = 25 days


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