an excerpt from net, hope this helps

Haemophilia is a recessive X linked hereditary disease so: (suppose X is the normal chromosome and x the one carrying the haemophilia gene)

-if the baby is a boy he has a 50% risk of having the disease. He will inherit the Y chromosome from his father so it's the mother who will determine if he will be sick or healthy. He can inherit X (50%) (XY: healthy) or x (50%) (xY: sick) from his mother.

-if it's a girl, she also has a 50% risk of having the disease. She will inherit x from her father (100%) so it's the mother who will determine if she will be sick or just a carrier. She will inherit either X (50%) (Xx: carrier) or x (50%) (xx: sick) from her mother. If she's sick she won't survive.

The baby can be a girl (50%) or a boy (50%) so the risk is:
50%x50% + 50%x50% = 50%

50/100*50/100+50/100*50/100

=25/100+25/100

=50/100