Q. A salesman's terms were changed from a flat commission of 5% on all his sales to a fixed salary of $1000 plus 2.5% commission on all sales exceeding $4000. If his remuneration as per the new scheme was $600 more than the first scheme, what were his sales worth?

This is a very tricky question....I can't just form the statements.

I don't even know what the constants are in this problem.

Thanks