Linda and Hazel

    18)  If it takes Linda 2.5 hours to clean the auditorium, but
Hazel can do it in 2, then how long would it take if they worked
together?

    Instead of using 1 / 2.5 for Linda, we'll convert it to 2 / 5.                         

    2       1      1
  ----  +  ---- = ----
    5       2      x

    Find the lowest common denominator:

    L.C.D. = 10x

    10x (2)    10x (1)      10x (1)
       ----  +    -----  =      ----
         5          2             x

    10x / 5 = 2x
    2x * 2 = 4x

    10x / 2 = 5x
    5x * 1 = 5x

    10x / x = 10
    10 * 1 = 10

    4x + 5x = 10

    9x  = 10

    x = 1.11 hours or 1 hour and 7 minutes.