5(3a²b - ab²) - (ab² + 3a²b)
= 15a²b - 5ab² - ab² - 3a²b
= 12a²b - 6ab²
= 6ab(2a-b)
= 6×1/2×1/3×(2×1/2- 1/3)
= 1×(2/3)
= 2/3