p : ℕ → Prop
inst✝ : DecidablePred p
n : ℕ
⊢ (Fintype.card ↑fun x => x < n ∧ p x) = Fintype.card { k // k < n ∧ p k }
p : ℕ → Prop
inst✝ : DecidablePred p
n : ℕ
⊢ count p n = Fintype.card { k // k < n ∧ p k }
rw [count_eq_card_filter_range, ← Fintype.card_ofFinset, ← CountSet.fintype]
no goals
p : ℕ → Prop
inst✝ : DecidablePred p
n : ℕ
⊢ (Fintype.card ↑fun x => x < n ∧ p x) = Fintype.card { k // k < n ∧ p k }
rfl