Class C, D and E were given some mochi balls to sell for their charity drive. The average number of mochi balls the 3 classes had at first was 215. After Class D sold four times as many mochi balls as Class C and Class E sold ⁷₈ as many mochi balls as Class D, the average number of mochi balls the 3 classes had left was 45. How many mochi balls did Class C and Class E sell altogether?

Class C	Class D	Class E
1x2	4x2
	8x1	7x1
2 u	8 u	7 u

Class C : Class D : Class E
2 : 8 : 7

Total number of mochi balls that the 3 classes had at first
= 3 x 215
= 645

Number of mochi balls that the 3 classes left
= 3 x 45 
= 135

Number of mochi balls that were sold by 3 classes
= 645 - 135
= 510

2 u + 8 u + 7 u = 510
17 u = 510

1 u = 510 ÷ 17 = 30

Total number of mochi balls sold by Class C and Class E
= 2 u + 7 u
= 9 u
= 9 x 30
= 270

Answer(s): 270