All human blooid is one of the four types : o , a, b or ab. the distributions of the types varies a bit with race. if you choose an african american person at random , here is a table of the approcimate probabilities of that person having each of the different blood types.

blood types : o a b ab
0.5 ? 0.2 0.05

using the table of probabilities above , the probability that the person chosen has blood type a is :
0.05
0.25
0.27
0.40
0.65

i'm having a bit of trouble with probability may someone walk me through it , i'm willing to learn!