Find last year’s salary if, after a 3% pay raise, this year’s salary is $30,900

Your answer is $30,000.
The way I have answered this is quite strange, but I'll do my best to explain it. So because we know that $30,900 is 3% than last year, we can call it 103%. This allows us to form a ratio and therefore find 100%.
30,900 : 103
÷ 103
300 : 1
× 100
30,000 : 100
Which means $30,000 is 100%, or 3% less than $30,900. I hope this helps! Let me know if it was confusing or anything :)