x and y can do a piece of work in 20 days and 12 days respectively. x started the work alone and then after 4 days y joined him till the completion of the work. how long did the work last ?

6 days

15 days

20 days

10 days

x and y can do a piece of work in 20 days and 12 days respectively. x started the work alone and then after 4 days y joined him till the completion of the work. how long did the work last ?

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