Solution:

A can build the structure in 8 days.

Fraction of structure built in a day by A =1/8

Similarly, fraction of structure broken by B in a day =1/3.

Amount of work done by A in 4 days 4/8=1/2.

Now, both A and B together for 2 days.

So, fraction of structure built in 2 days =2(1/8−1/3)=−5/12

Fraction of structure still to be built =1/2+5/12=11/12.

If A takes x days to build up the remaining structure, then:x/8=11/12

⇒x= 22/3 days.          

Solution:

Let 'a' hours be the time that worker A will take to complete the job. 

Let 'b' hours be the time that worker B takes to complete the job. 

When A works for 2 hours and B works for 5 hours half the job is done. 

i.e. 2/a+5/b=1/2 ------------ (1)

When they work together for the next three hours, (1/20)th of the job is yet to be completed. 

They have completed half the job earlier and (1/20)th is still left. 

So by working for 3 hours, they have completed 1–{1/2}−1/20=9/20 of the job. 

Therefore, 3/a+3/b=9/20 ------------- (2). 

Solving equations (1) and (2), we get b = 15 hours.

Solution:

Expenses proportional to (Number of acres ploughed per day) (Number of days).

Assuming A used the tractor for 'd' days, the ratio of A's to B's expenditure is:

⇒ 12d:(23−d)15

This is given to be equal to 3000:2000 or 3:2

Thus, 12d:(23−d)15=3:2

Thus d=15.

Therefore A's used the tractor for 15 days.

Solution:

Let it takes x days to complete the work

The A worked for (x−2) days, B for (x−4) days and C's for x days.

x/40+x−4/30+x−2/15=1

⇒ x=152/15

Solution:

n(1/12+1/16)=28n/192

⇒ Left capacity =1–28n/192

This is filled by A in 5 min and fills 1/12 in 1 min

⇒ (192−28x)/192=5/12

⇒ n= 4 min