car(ford, focus, 2014, 60000, 7500).
car(ford, mondeo, 2011, 120000, 3000).
car(bmw, 5-series, 2013, 75000, 11000).
car(volvo, 850, 1995, 250000, 1000).
car(bmw, 3-series, 2014, 60000, 15000).
car(roles_royce, special, 1950, 120000, 100000).

show_cars_age_and_price(Make) :-
     car(Make, Type, Y, _, P),
     write(Make), write(' '), write(Type), write(', '),
     Age is 2017 - Y, write(Age), write(' years old, '),
     write(' Price is '), write(P), nl,
     fail;
     write('No more to display').


  avg_car_cost :-
    retractall(sum(_)),
    retractall(count(_)),
  	assert(sum(0)),
  	assert(count(0)),
  	compute,
  	sum(X),
  	count(N),
  	Avg is X / N,
  	write('Average price is '), write(Avg), nl,
  	retract(sum(X)),
  	retract(count(N)).

 compute :-
     car(_, _, _, _, Cost),
     sum(X), count(C), 
     X1 is X + Cost, C1 is C + 1,
     retract(sum(X)),
  	 retract(count(C)),
  	 assert(sum(X1)),
  	 assert(count(C1)),
  	 fail;
  	 true.