class A
  def initialize a
    @arr = a
  end
  def get i
    @arr[i]
  end
  def sum
    @arr.inject(0) {|acc,x| acc + x}
  end
end

class B < A
  def initialize a
    super
  end
  def sum
    @and ||= super
  end
end


v = [4,19,74]
a = A.new v
b = B.new v
s1 = a.sum
s2 = b.sum
...
s3 = a.sum
s4 = b.sum

Have the initialize method in class A store a copy of its argument in @arr.
Remove the method get from class A.     ✘     0.00
Change the sum method in both classes to use an explicit loop instead of inject and a block.
Change class A to use a class variable @@arr in place of the instance variable @arr.