Answer:
6 plates are not used. The sum of the 9 plates used for the party and the 6 plates not used equals 15. Thus, subtracting 9 from 15 allowed us to determine that 6 plates were not used.
Explanation:
We know that of Marta's 15 dinner plates:
- 9 are used for a party,
- and the remaining number of plates are not used.
We can determine how many plates are not used by subtract 9 from 15:
Plates not sued = 15 - 9
Plates not used = 6
Thus, 6 plates are not used.