What blood types are possible for making blood type A?

Funny, I have a problem just like this one in my Genetics book, but you don't have to do alot of calculations like I did. Here's how to solve it.

1) It's obvious that the father didn't help conceive the child. The father has a B blood type, the mother has a O blood type, and the child has an A blood type. The only way the mother would have an A blood type child is if she laid down with a man with blood type A. If it was from the father, then the only two blood type the child would have is either B or O.

2) If you do a Punnett square of AiBi x Bi ii, you get: AB, BB, Ai, and Bi. So you get a blood type AB at a probability of 1/4 (25%), a blood type B at a probability of 2/4 (50%), a blood type A at a probability of 1/4 (25%), and a blood type O at a probability of 0/4 (0%)