Since 45 is 9 times 5, it is 9 times as likely that someone will walk
in when she is working than when she is on break.

Let p = the probability that someone will walk in while she is on break.
Then 9p = the probability that someone while walk in while she is working.

p + 9p = 1
   10p = 1
     p = 1/10 = the probability that someone will walk in which she is on break.

Therfore the prabability that someone will walk in while she is working is

9p = 9(1/10) = 9/10 or 90% of the time.

Edwin