David had three boxes, containing a total of 880 styrofoam plates. The number of styrofoam plates in Box B to the total number of styrofoam plates was 3 : 11. He sold 341 styrofoam plates from Box C and sold ¹₅ of the styrofoam plates in Box D. The number of styrofoam plates left in Box C to the number of styrofoam plates left in Box D was 2 : 1. How many styrofoam plates were there in Box C at first?

	Box B	Box C	Box D	Total
Before	3 u	8 u (640)		11 u (880)
		8 p + 341	5 p
Change		- 341	- 1 p
			4 p
After		2x4 = 8 p	1x4 = 4 p

11 u = 880
1 u = 880 ÷ 11 = 80

Number of styrofoam plates in Box C and Box D at first
= 8 u
= 8 x 80
= 640

The number of plates left in D is the repeated identity.
LCM of 1 and 4 = 4

8 p + 341 + 5 p = 640
8 p + 5 p = 640 - 341
13 p = 299

1 p = 299 ÷ 13 = 23

Number of plates in Box C at first
= 8 p + 341
= 8 x 23 + 341
= 184 + 341
= 525

Answer(s):525