Scanning a txt file with Java?

I know how to read a txt file in Java but what I dont know how to do is scan a txt file in java based on a starting value a user gives me.

For example: Say I have a txt file with a list from 1-20. ANd after each of those numbers is a answer to a math problem. Sure I can read it normally but how do I go about getting everything after that one number the user gave me? I just made this up below but you should get the idea.

A user inputs the number 1
The program scans the txt file for the first 1 that appears
The program then outputs the rest of the line after 1.

1. 5460
2. 8475
3. 4850
4. 9121

Performing IO is expensive (resource wise). So you want to read the text file once and store all the data in a Collection. Afterwards you can search/sort your data which is much simpler.