Find the probability of the the sum 
A new drug has been released and produces some minor side effects. 8% of users suffer
  only a loss of sleep and 12% of users suffer only bouts of nausea. 75% of users will have
  no side effects at all. What percentage of users will suffer from both loss of sleep and
  nausea?

What I learn in school  is: if there is or in question then sum ($+$) , if there is and then multiply ($\times$)

Ans: $12*8=96$ 

and I have try many ways, I can't get the right answer 5% 
I have no idea how to find the probability, please explain me.
Highly appreciate it. thanks

You just need to add these numbers so out of 100  - (8 + 12 + 75) = 100 - 95 = 5%.
Essentially we are accounting for percentage of people out of a group having a particular property, whatever is left are the bunch who have both since every one else who has been counted has only one specific property.
