It takes Brenda 15 hours to clean an attic. Danielle can clean the same attic in 13 hours. Find out long it would take them if they worked together.

hours to complete job:
  Brenda: 15
  Danielle: 13
  together: t

 

completed per hour:
  Brenda: 1/15
  Danielle: 1/13
  together: 1/t

 

adding their labor:

1/15 + 1/13 = 1/t

28/195 = 1/t

195/28 = t

T= approx. 7 hours