apply Finset.card_image_of_injOn
C : Type u_1
J : Type u_2
r : C → J → Prop
inst✝¹ : Fintype J
inst✝ : Fintype C
p : JudgePair J
h : p.Distinct
⊢ (Finset.image AgreedTriple.contestant (Finset.filter (fun a => a.judgePair = p) (A r))).card =
    (Finset.filter (fun a => a.judgePair = p) (A r)).card
case H
C : Type u_1
J : Type u_2
r : C → J → Prop
inst✝¹ : Fintype J
inst✝ : Fintype C
p : JudgePair J
h : p.Distinct
⊢ Set.InjOn AgreedTriple.contestant ↑(Finset.filter (fun a => a.judgePair = p) (A r))