за 3 5/21 часа.
7 + 2 = 9.
1 : 7 = 1/7
1 : 9 = 1/9
1/7 + 1/9 = 9/63 + 7/63 = 16/63
1 : 16/63 = 63/16 = 3 15/63 = 3 5/21
за 3 5/21 часа.
7 + 2 = 9.
1 : 7 = 1/7
1 : 9 = 1/9
1/7 + 1/9 = 9/63 + 7/63 = 16/63
1 : 16/63 = 63/16 = 3 15/63 = 3 5/21