What year is it if it is the 6th year of the 3rd decade in the 5th century AD?

Question

What year is it if it is the 6th year of the 3rd decade in the 5th century AD?

 

My answer is 425 but the answer is 426.

I think 6th year is 5(start from 0)

Answer 1

The fifth century started on year 401 ( there is no year 0 ).  The third decade of any century starts on year 21. The sixth year of any decade is 6 ( count from 1 to six and you'll see) .So the sixth year of the third decade of the fifth century is 426. 

Answer 2

The 5th century is in between the year 401 and 500.

 

A decade is equal to 10 years.  The year in the third decade in the 5th century is 431.

 

Six years after the year 431 is the year 437 because it is in the 3rd decade.  425 is in the 2nd decade.

 

The year is 437.