n = 500, 
p = .5,
q = 1-.5 = .5,
x = 251.5 

(Since it included 251 we add .5, if it had not
 included 251 we would have subtracted .5)







So look up 0.13 in a standard normal table.

There are two kinds of tables.

In one kind of table you will read 0.0517 and you have to add .5
to get 0.5517.

In the other kind of table you will get 0.5517 directly.

Edwin