Problem Solving

For any four digit number, abcd, *abcd*= (3a)(5b)(7c)(11d). What is the value of (n – m) if m and n are four-digit numbers for which *m* = (3r)(5s)(7t)(11u) and *n* = (25)(*m*)?

2000
200
25
20
2


qa is 200. I can follow all the steps but what can i take away from this. The approach is the hardest part. How can i get better at this?