Day -9 Dr. Sahab Series

Dr. Robert Edwards , Dr . Patrik Steptoe and Dr. Indira Hinduja are famous for their work on

Dr.M.S. Swaminathan is associated with

The 9^(th) term of the series 27+9+27/5+27/7+...

Which term of the series 21, 15, 9, ... is -39 ?

A and B can do a piece of work in 6 days and 9 days respectively, they work on alternative days starting with A on the first day. In how many days will the work be completed ?