Digging companies

Company A would dig a pit for 12 days, company B for 15 days, company C for 20 days, and company D for 24 days. Work began together with companies C and D but after three days. They joined the other two companies. How long took to dig a pit?

Correct answer:

x =  6 d

Step-by-step explanation:

A=12 B=15 C=20 D=24  x(1/C+1/D) + (x3)(1/A+1/B) =1  A B x (D+C)+C D (x3) (B+A)=A B C D 12 15 x (24+20)+20 24 (x3) (15+12)=12 15 20 24  20880x=125280  x=20880125280=6  x=6=6 d



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