X and Y can complete a project in 20 and 12 days, respectively. X began working alone, but after four days, Y joined him and worked with him until the job was completed. What was the duration of the project?
A. 6 days
B. 10 days
C. 15 days
D. 20 days
Answer:B
X and Y can complete a project in 20 and 12 days, respectively. X began working alone, but after four days, Y joined him and worked with him until the job was completed. What was the duration of the project?
A. 6 days B. 10 days C. 15 days D. 20 days