1. She could get 9 envelopes for 4 dollars each (or the other way around)
2. She could get 12 envelopes for 3 dollars each (or the other way around)
3. She could get 18 envelopes for 2 dollars each (or the other way around)
If you only want to know what it means ¨c¨ then it is the Denominator, cause you are referring a fraction.