Ahmad and Kumar together can paint a house in 12 days. Kumar and Chong Beng together can complete the same job in 15 days while Ahmad and Chong Beng together will take 20 days to paint the same house. How many days will it take Ahmad, Kumar, and Chong Beng to complete the job together?

Ahmad and Kumar together can paint a house in 12 days. Kumar and Chong beng together can complete the same job in 15 days while Ahmad and Chong beng together will take 20 days to paint the same house. How many days will it take Ahmad, Kumar and Chong beng to complete the job together?

The trick (SOP) for work problems is that we reduce the work to unit rate.

Ahmad and Kumar together can paint a house in 12 days.--->Ahmad and Kumar together can paint 1/12 the house in 1 days.

A+K = 1/12 of house in a day

K+C = 1/15 of house in a day

A+C = 1/20 of house in a day

Adding all together

2 (A+K+C) = 1/12+1/15/+1/20 = 1/5 in a day

A+K+C = 1/10 in a day

So they take 10 days to finish the job together.