A piece of work can be done by NAME_1 NAME_2 in 20 days, NAME_2 and NAME_3 in 15 days and NAME_1 NAME_3 in 20 days.  Calculate and write how many days the three will complete the work together.  If NAME_4, NAME_2 and NAME_3 work separately, we calculate who can do the work in how long. Provide your reponse without using the word name_3.
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