task about CTR

#1
Hi guys!
I have a question:
There is link on the main google page with CTR 1.5% (CTR is number of clicked visitors divided by number of all visitors). The link was changed. And then google began to show new link in 4% of cases (and previous link in 96%, which has CTR 1.5%). CTR of new link is unknown. How many shows of main Google page we need to find out about changes of CTR with accuraccy 95%?