Question

The rational numbers which lie exactly at midway between 3.456 and 3.457

Answer 1

there is an infinite number of rational number in between two rational numbers

so you cannot possibly tell a number to be in the middle of two numbers
some numbers can be 3.4560, 3.4561..... many more.

Answer 2

There are too many rational numbers between 3.456 and 3.457,some of them possibly are 3.4560,3.4561,3.4562,3.4563 and so on.