А) 3,1(28) + 2,(21) = 3 127/990 + 2 210/990 = 5 337/990 = 5,3(40)
3,1(28) = 3 127/990 2,(21) = 2 21/99
10х = 31,2828... х = 2,2121...
1000х = 3128,2828... 100х = 221,2121...
990х = 3128 - 31 99х = 221 - 2
х = 3097/990 = 3 127/990 х = 219/99 = 2 21/99 = 2 210/990