Independently, A does 1/15 of the work in one day, B does 1/20 of the work in one day & C does 1/30 of the work in one day.
1. If they work for an equal number of days, the salary of A & B will be in the ratio of the rate of their work per day i.e. Salary of A:B = (1/15)/(1/20) = 4/3
(Notice that A works the best as he is able to complete the work fastest compared to others or in other words in one day, A does a higher amount of work compared to say, B. Hence, he is also expected to get a better salary compared to B or C).
From pt. 1, let's say, A gets 4x & B gets 3x salary in the entire period they worked till completion (this holds good since both of them worked for an equal number of days). Therefore, C gets, 3x - 6000 salary since it is given in the question that B gets 6000 more salary C.
2. Salary of A+B+C = 4x + 3x + (3x-6000) = 18000 (from the question) => x = 2400
From pt. 2, it follows that C got 3x2400 - 6000 = 1200 salary
Till the time A, B & C worked together, C got 1200 salary & now let's try to figure out how much salary did A & B get in the same time frame. To arrive at the same, note that the ratio of salary of B:C (same logic from pt. 1)= (1/20)/(1/30) = 3/2 & hence, B would have got 3/2 x 1200 = 1800 salary & A would have got 4/3 x 1800 = 2400 salary. So, in the first phase of work when all of them worked together, total salary would amount to: 2400+1800+1200 = 5400
This essentially means in the first phase of all working together, (5400/18000) = 3/10th of the work got completed & 7/10th of the work remains to be completed.
No of days (n) it must have taken to complete 3/10th of the work (1st phase):-
n.[1/15 + 1/20 + 1/30] = 3/10
=> n = (3/10)/[(4+3+2)/60] => n = (3/10)/(9/60) =
2In the 2nd phase when only A & B worked together & 7/10th of the work got completed, no of days (n') it must have taken to complete this work:-
n'.[1/15+1/20]=7/10
=>n'=(7/10)/[(4+3)/60] =>n'=(7/10)/(7/60) =
6Since A worked for complete duration across phase 1 & 2, total number of days A worked is n+n' = 2+6 =
8.
So, answer is Option D.
Thanks
Devneet