4) 3 5/21.
Пошаговое объяснение:
8/7 • 2 5/6 = 8/7 • (2•6+5)/6 = 8/7 • 17/6 = (8•17)/(7•6) = (4•17)/(7•3) = 68/21 = 3 5/21.
