theorem extracted_formal_statement_12.{u_1, w, u} {Ω : Type u} {Z : Type w} {n : ℕ} {ι : Type u_1} {f : ι → Z → ℝ}
  [inst : Nonempty ι] {X : Ω → Z} {b : ℝ} (hf' : ∀ (i : ι) (z : Z), |f i z| ≤ b) (ω : Fin n → Ω × Ω)
  (h_original_goal :
    (↑(Fintype.card (Signs n)))⁻¹ * ∑ σ, ⨆ i, |∑ k, ↑↑(σ k) * (f i (X (ω k).1) - f i (X (ω k).2))| ≤
      ((↑(Fintype.card (Signs n)))⁻¹ * ∑ σ, ⨆ i, |∑ k, ↑↑(σ k) * f i (X (ω k).1)|) +
        (↑(Fintype.card (Signs n)))⁻¹ * ∑ σ, ⨆ i, |∑ k, ↑↑(σ k) * f i (X (ω k).2)|) :
  (↑(Fintype.card (Signs n)))⁻¹ * ∑ σ, ⨆ i, |∑ k, ↑↑(σ k) * (f i (X (ω k).1) - f i (X (ω k).2))| ≤
    (↑(Fintype.card (Signs n)))⁻¹ *
      ((∑ σ, ⨆ i, |∑ k, ↑↑(σ k) * f i (X (ω k).1)|) + ∑ σ, ⨆ i, |∑ k, ↑↑(σ k) * f i (X (ω k).2)|) := sorry

State before:
Ω : Type u
Z : Type w
n : ℕ
ι : Type u_1
f : ι → Z → ℝ
inst✝ : Nonempty ι
X : Ω → Z
b : ℝ
hf' : ∀ (i : ι) (z : Z), |f i z| ≤ b
ω : Fin n → Ω × Ω
⊢ (↑(Fintype.card (Signs n)))⁻¹ * ∑ σ, ⨆ i, |∑ k, ↑↑(σ k) * (f i (X (ω k).1) - f i (X (ω k).2))| ≤
    ((↑(Fintype.card (Signs n)))⁻¹ * ∑ σ, ⨆ i, |∑ k, ↑↑(σ k) * f i (X (ω k).1)|) +
      (↑(Fintype.card (Signs n)))⁻¹ * ∑ σ, ⨆ i, |∑ k, ↑↑(σ k) * f i (X (ω k).2)|

Tactic: rw [← mul_add]

State after:
[Ω : Type u
 Z : Type w
 n : ℕ
 ι : Type u_1
 f : ι → Z → ℝ
 inst✝ : Nonempty ι
 X : Ω → Z
 b : ℝ
 hf' : ∀ (i : ι) (z : Z), |f i z| ≤ b
 ω : Fin n → Ω × Ω
 ⊢ (↑(Fintype.card (Signs n)))⁻¹ * ∑ σ, ⨆ i, |∑ k, ↑↑(σ k) * (f i (X (ω k).1) - f i (X (ω k).2))| ≤
     (↑(Fintype.card (Signs n)))⁻¹ *
       ((∑ σ, ⨆ i, |∑ k, ↑↑(σ k) * f i (X (ω k).1)|) + ∑ σ, ⨆ i, |∑ k, ↑↑(σ k) * f i (X (ω k).2)|)]

Relevant lemmas:
mul_add : ∀ {R : Type v} [inst : Mul R] [inst_1 : Add R] [inst_2 : LeftDistribClass R] (a b c : R),
  a * (b + c) = a * b + a * c