Class X, Y and Z were given some mochi balls to sell for their fund-raising project. The average number of mochi balls the 3 classes had at first was 239. After Class Y sold twice as many mochi balls as Class X and Class Z sold ²₃ as many mochi balls as Class Y, the average number of mochi balls the 3 classes had left was 57. How many mochi balls did Class X and Class Z sell altogether?

Class X	Class Y	Class Z
1x3	2x3
	3x2	2x2
3 u	6 u	4 u

Class X : Class Y : Class Z
3 : 6 : 4

Total number of mochi balls that the 3 classes had at first
= 3 x 239
= 717

Number of mochi balls that the 3 classes left
= 3 x 57 
= 171

Number of mochi balls that were sold by 3 classes
= 717 - 171
= 546

3 u + 6 u + 4 u = 546
13 u = 546

1 u = 546 ÷ 13 = 42

Total number of mochi balls sold by Class X and Class Z
= 3 u + 4 u
= 7 u
= 7 x 42
= 294

Answer(s): 294