Answer to Problem #20:

This is a 'parallel' problem solved by a formula (derived in the text):

   1    =   1    +    1      where: Tt   =   time for job (together)
  Tt        T1       T2                  T1   =   time for job (1st person alone)
                                               T2   =   time for job (2nd person alone)

To simplify computation, the above formula can be reworked to:

                               Tt    =   T1 x T2         
                                            T1 + T2

      30  =    45 x T2      45 T2   =    30 (45 + T2)    =    1350 + 30 T2
                   45 + T2 

      45 T2   -  30 T2    =   1350        15 T2    =   1350       T2 = 1350 / 15

                                   T2    =   90 days (ANSWER)


 
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