1) 5 (3 boys + 2 girls)
                2) 41
                3) Archie:    10
                   Barry:     14
                   Charlie:   17

    Notes

  2). Total age of girls       =  15 X 3  =  45  
      Total age of family      =  25 X 5  = 125
      So total age of parents  = 125 - 45 =  80
      So father is 41 (and mother is 39) 

  3). Total age:          A + B + C = 41
      C-A years ago:      C - A = B/2
      C-B years ago:      A - (C - B) = C - 10
      Solving:      A = 10, B = 14, C = 17

  1).A fraction (such as 152/364) has a three-digit number as
      numerator and a three-digit number as denominator. What is
      the largest such fraction (smaller than 1) that you can
      form using each of the digits 1, 2, 3, 4, 5, 6 once only?

  2).If John gave one-third of his money to David, then they would
      both have the same. But if, instead, David gave one-third of
      his money to John, then John would have £80 more than David.
      How much money has each?

  3).At noon I set off to drive the 250 miles to London to visit my
      friend, driving at 60 miles per hour. But half an hour later
      my friend set off from London to visit me, driving at 50 miles
      per hour. How far from London did we pass each other?