Bunuel wrote:
A contractor undertakes to do a job within 100 days and hires 10 people to do it. After 20 days, he realizes that one fourth of the work is done so he fires 2 people. In how many more days will the work get over?
(A) 60
(B) 70
(C) 75
(D) 80
(E) 100
Kudos for a correct solution.
VERITAS PREP OFFICIAL SOLUTION:Can we say that 10 people can finish the work in 100 days? No. If that were the case, after 20 days, only 1/5th of the work would have been over. But actually 1/4th of the work is over. This means that ‘10 people can complete the work in 100 days’ was just the contractor’s estimate (which turned out to be incorrect). Actually 10 people can do 1/4th of the work in 20 days. The contractor fires 2 people. So the question is how many days are needed to complete 3/4th of the work if 8 people are working?
We need to find the number of days. How is ‘no. of days’ related to ‘no. of people’ and ‘work done’?
If we have more ‘no. of days’ available, we need fewer people. So ‘no. of days’ varies inversely with ‘no. of people’.
If we have more ‘no. of days’ available, ‘work done’ will be more too. So ‘no. of days’ varies directly with ‘work done’.
Therefore,
‘no. of days’ * ‘no. of people’/’work done’ = constant
\(20*\frac{10}{(\frac{1}{4})} =\) ‘no. of days’*\(\frac{8}{(\frac{3}{4})}\)
No. of days = 75
So, the work will get done in 75 days if 8 people are working.
We can also do this question using simpler logic. The concept used is joint variation only. Just the thought process is simpler.
10 people can do 1/4th of the work in 20 days.
8 people can do 3/4th of the work in x days.
Start with the no. of days since you want to find the no of days:
\(x = 20*(\frac{10}{8})*(\frac{3}{1}) = 75\)
From where do we get 10/8? No. of people decreases from 10 to 8. If no. of people is lower, the no of days taken to do the work will be more. So 20 (the initial no. of days) is multiplied by 10/8, a number greater than 1, to increase the number of days.
From where do we get (3/1)? Amount of work increases from 1/4 to 3/4. If more work has to be done, no. of days required will be more. So we further multiply by (3/4)/(1/4) i.e. 3/1, a number greater than 1 to further increase the number of days.
This gives us the expression \(20*(\frac{10}{8})*(\frac{3}{1})\)
We get that the work will be complete in another 75 days.
Answer (C)