Question

In a coaching centre 70% of the students knew course A,60% of them knew course B ,10% of them don't know both the courses. How many of them knew both the courses. ​

Answer 1

Total students =who only know course A+who only know course B+who know both A&B courses +who dont know any coursr

let who know both courses be x%

hence,who know only A=70%-x%

who know only course B=60%-x%

now,

Total students constitute 100%

100%=[70%-x%]+[60%-x%]+x%+10%

thus x=40%

This problem is more clear with venn diagram..