Originally Posted by websiex

You get the 8 shots you take and divide them by the automatic 20%[.2](heads) that is going to take place. Now, you want to see how many times that 20% (out of 8) is going to happen exactly 2 times, so you set it equal to 2. So you get:

8/.2=2
40=2
40=40
2/40= .05

.05%