simp_rw [Finset.sum_mul, div_mul_eq_mul_div, one_mul]
⊢ (∑ i ∈ Finset.range 330, 1 / ((660 + ↑i) * (1319 - ↑i))) * ↑(∏ j ∈ Finset.range 330, (660 + j) * (1319 - j)) =
    ↑(∑ i ∈ Finset.range 330, ∏ j ∈ (Finset.range 330).erase i, (660 + j) * (1319 - j))
⊢ ∑ x ∈ Finset.range 330, ↑(∏ j ∈ Finset.range 330, (660 + j) * (1319 - j)) / ((660 + ↑x) * (1319 - ↑x)) =
    ↑(∑ i ∈ Finset.range 330, ∏ j ∈ (Finset.range 330).erase i, (660 + j) * (1319 - j))