1023.
2 : 5 1/3 = 2 : 16/3 = 2 * 3/16 = 3/8,
2 4/5 : 7 = 14/5 * 1/7 = 2/5,
12 : 2 2/3 = 12 : 8/3 = 9/2 = 4 1/2,
21 1/4 : 5 = 85/4 * 1/5 = 17/4 = 4 1/4,
1024.
1 1/5 : 1/5 = 6/5 * 5 = 6,
2 2/5 : 2/3 = 12/5 * 3/2 = 18/5 = 3 3/5,
5/9 : 4 1/6 = 5/9 : 25/6 = 5/9 * 6/25 = 2/15,
5/6 : 3 1/3 = 5/6 : 10/3 = 5/6 * 3/10 = 1/4,
2 1/2 : 1 2/3 = 5/2 : 5/3 = 5/2 * 3/5 = 3/2 = 1 1/2,
3 1/2 : 1 3/4 = 7/2 : 7/4 = 7/2 * 4/7 = 4/2 = 2,
2 2/5 : 1 1/5 = 12/5 : 6/5 = 12/5 * 5/6 = 12/6 = 2,
5 2/5 : 1 1/9 = 27/5 : 10/9 = 27/5 * 9/10 = 243/50 = 4 43/50