1.  14 yr.
    2.  34.2 m
    3.  212
   
   Notes
 1. Let a = Mr. Jones' age, b = Mrs. Jones' age, m = years married.
    
    b     = 6/7 a                So 7b = 6a
    b + 7 = 7/8 (a + 7)          So 8b + 56 = 7a + 49
      Solving, a = 49, b = 42

    So 42 - m = 4/5 (49 - m)
      Solving,  m = 14 

 2. Let x cm = spacing, n = no. of posts, l cm = length of fence.
    
    So l = (n - 1)x
       l = (n - 2)(x + 10)
       l = n(x - 9)
    Solving, n = 20, x = 180, l = 3420

 3. By intelligent trial and error:
      Andy:     Guess = 236:   Error = 24
      Beatrice: Guess = 200:   Error = 12
      Charlie:  Guess = 220:   Error =  8