((15/28-11/36)*21/29+6/67:16/21):16/12 =
1) 15/28 - 11/36 = 135/252 - 77/252 = 58/252 = 29/126,
2) 29/126 * 21/29 = 21/126 = 6,
3) 6/67 : 16/21 = 6/67 * 21/16 = 63/536,
4) 6 + 63/536 = 6 63/536,
5) 6 63/536 : 16/12 = 3279/536 * 12/16 = 9837/2144 = 4 1261/2144