Решение:
(3,5 ⋅ 10⁻⁵) ⋅ (6,4 ⋅ 10²) = (3,5 ⋅ 6,4) ⋅ (10⁻⁵ ⋅ 10²) = 22,4 ⋅ 10⁻³ = 2,24 ⋅ 10¹ ⋅ 10⁻³ = 2,24 ⋅ 10⁻².
Ответ: 2,24 ⋅ 10⁻²