probability with random variables

About 25% of the people called upon for jury duty give an excuse not to serve. If 12 ppl are called for duty...

a) What is the prob. that 8 or more will not be able to serve on the jury?

this is wat i did..

1-(12|0 ,.25^0, .75^12)+ ....and do the binomial for 0,1,2,3,4,5,6,7 and add then up. For my answer, i got .649.

__Is this right?

b)How many people must the jury commissioner contact to be 95% sure of finding at least 12 ppl who are available to serve?

__How would I do this?