3(4/9) - 1(16/21) = 3(28/63) - 1(48/63) = 1(43/63).
1(43/63) : 1(11/42) = 106/63 * 42/53 = 4/3 = 1(1/3).
1(5/7) : 6 = 12/7 * 1/6 = 2/7.
3(4/9) - 1(16/21) = 3(28/63) - 1(48/63) = 1(43/63).
1(43/63) : 1(11/42) = 106/63 * 42/53 = 4/3 = 1(1/3).
1(5/7) : 6 = 12/7 * 1/6 = 2/7.