Why Traffic and Search Engine position for my webisite Dropped

I have an educational website , I was ranked on first page of google for most of my targetted keywords in first half of may, After 23rd May update , my traffic dropped , I never did 

•  Black hat seo 
  
• no spammy links 
  
• never purchased any link .
  

 Followed all webmaster guidelines , Still I could have manged with this traffic drop , if sites  better then  my site are placed above me ,By better I mean

• better in terms of content  
  
• in terms of on-site SEO 
  
•  in terms of link buliding 
  

Please write Computer Networking MCQ and check in google , on what basis google is listing sites .

am not able to understand, feeling really disappointing ,

same thing happened for all of my keywords.

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