a can do a piece of work in 10 days, and b can do the same work in 20 days. with the help of c, they finished the work in 4 days. c can do the work in how many days, working alone?

a can do a piece of work in 10 days, and b can do the same work in 20 days. with the help of c, they finished the work in 4 days. c can do the work in how many days, working alone?