Sam and Dave

    16) If Sam can rake the leaves in the front yard in 35 minutes,
and Dave can do the same job in 40 minutes, then how long would
it take if they work together?

   1      1      1
  --- + ----- = ---
   35    40      x

    Find the least common denominator:

     35 =             5 * 7
     40 = 2 * 2 * 2 * 5
          -----------------
          2 * 2 * 2 * 5 * 7 = 280

        L.C.D. = 280x

        280x (1)     280x (1)      280x (1)
             ---  +       ----  =       ---
              35           40            x

    280x/35 = 8x
     8x * 1 = 8x

    280x/40 = 7x
      7x*1 = 7x

    280x/x = 280
    280 * 1 = 280

    8x + 7x = 280

    15x = 280

    x = 18.66 minutes, or 18 minutes and 40 seconds.