a:b=2:7, b:c=3:1, c:d=2:7a:b=6:21, b:c=21:7, c:d=2:7a:b=6:21, b:c=21:7, c:d=14:49a:b=6:21, b:c=42:14, c:d=14:49a:b=12:42, b:c=42:14, c:d=14:4912:42:14:49