Then the mare should be replaced by another horse.

And it is an integer head, then the pony is an even head.

According to the minimum consideration, five mares can be exchanged for three stallions and two ponies.

A similar 10 mare can be exchanged for 6 stallions and 4 ponies.

15 mares can be exchanged for 9 stallions and 6 ponies.

20 mares can be exchanged for 12 stallion and 8 ponies.

......

Substitution in the same way is the answer (only the multiple of 10 is quoted)

Male: female: small = (0: 100:0? ) 6:90:4 12:80; 8 18:70: 12 24:60: 16 30:50; 20 36:40:24 42; 30:28 48; 20:32 54: 10:36 (60:0:40? )