25 men worked for 20 days and did 25*20 = 500 units of work
40 men worked for 15 days (the bridge was needed to be completed in 40 days but got done 5 days earlier) and did 40*15 = 600 units of work

So total work was 1100 units of work.
Had 25 men been there for the entire time, time taken = 1100/25 = 44 days

So he would have been 4 days behind the scheduled time.

Answer (C)
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25 men*20 days=500 total man days for original crew
40 men*15 days=600 total man days for enhanced crew
500/(500+600)=5/11 of job completed by original crew
let r=daily rate of original crew
r*20=5/11
r=1/44
44-40=4 days behind
C

Given: Originally, the work was to be completed by 25 workers in 40 days. If we assume the total
work a worker does in a day to be 1 units, the total work must be 1000(40*25) units.

In second statement of question stem we are given "After 20 days he appointed 15 more workers
and the bridge was completed 5 days earlier" - Here, we need to know how many workers actually
did the job.

x workers work for 20 days and x+15 work for 15 days to complete the work in 35 days.
\(x*20+ (x+15)15 > 1000\) -> \(35x > 775\) -> \(x > 22\). Minimum workers must be 23

Now, 23 workers complete 23*40 units or 920 units of work in 40 days.
Incomplete work: 80 units | Therefore, it will take 4(Option C) days in order to complete the pending work.
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Suppose, 1 man's 1 days' work = x

Now, 25*x*20+ 40*x*15 = 1
=>x = 1/1100

Again,
suppose, y days needed to complete whole work

Now,

25*y/1100=1

=> y = 44

So, extra days= 4 (ans)

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