i am not even able to understand the medium level problem in hackerearth

Question

can some one explain the mathematical logic behind the hackerearth problem of
"best index" under the basic input output section

the link is here -

https://www.hackerearth.com/practice/basic-programming/input-output/basics-of-input-output/practice-problems/algorithm/best-index-1-45a2f8ff/

Answer 1

please link the problem at least