conditional probability

Hi, i have a question about conditional prob.
i need to calculate the following prob. p(z1<a/z>=a)
while for example
z1=[1,1,2,1.98,3,3,3,4,3.98,3.97]
z=[1,1,2,2,3,3,3,4,4,4]
and a changes from 1 to 4
anyone can help me???
i tried using
p(z1<a/z>=a)=P(z1<a,z>=a)/P(z>=a)=(P(z>=a/z1<a)*P(z1<a))/P(z>=a)
but i'm not sure i do the right thing.
thanks.

.