Five years ago the average of A and B was 15 years Average age of A, B and C today is 20 years how old will C be after 10 years.

A) 10 years

B) 20 years

C) 30 years

D) 40 years

Five years ago ages of A and B are A – 5 , B – 5

So Averge ,  (A + B)/ 2 = 15

                     [(A – 5) + (B – 5)]/2 = 15

                      A + B – 10 = 30 

                      A + B = 40 ……….(i) 

Now average ages of A , B and C is 20 

                    ( A + B + C)/3 = 20 

                      A + B + C = 60 ……… (ii) 

Putting equation (i) into (ii) 

                           40 + C = 60 

                                    C = 20

After 10 years  C = 20 + 10 = 30

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