According to a marketing research firm, 52% of all residential telephone numbers in Los Angeles are unlisted. A telephone sales firm uses random digit dialing equipment that dials residential numbers at random, whether or not they are listed in the telephone directory. The firm calls 500 numbers in Los Angeles.

What is the probability that at least half of the numbers dialed are unlisted?

This is one of the questions that was on a bonus quiz I took. I got it wrong and I've been trying and trying to figure it out and I just can't. I'm using a calculator, the normalcdf function so I need to know the lowerbound, upperbound, mean, and standard deviation but every time I put in what I think is the correct information, I get 0.

Here's what I was inputting:

lowerbound : 250

upperbound : 10000

mean : .52

standard deviation : .0223427841

I went over a problem the Professor did earlier and did everything she did step by step so I'm not sure if one of my figures is wrong or if I'm using the wrong function. Maybe I should be using binomialcdf? I just tried it and I'm still not getting the right answer. The only possible answers are :

.5166

.4999

.5080

.8146

I guessed the first one after trying for so long that I was nearly out of time and it was wrong so we're down to .4999, .5080, or .8146.