My friend translated this problem from his native language, and I am unsure of the accuracy of the translation. If necessary, please edit to correct any faults.
A mineral substance, to be sent to a richening factory, contains $9\%$ metal. After maintenance, the substance contains $42\%$ metal, and the leftover contains $2\%$ metal. What percent of the substance becomes leftover?
How shall one proceed in order to solve this?