Class D, E and F were given some mochi balls to sell for their donation drive. The average number of mochi balls the 3 classes had at first was 216. After Class E sold twice as many mochi balls as Class D and Class F sold ⁶₇ as many mochi balls as Class E, the average number of mochi balls the 3 classes had left was 51. How many mochi balls did Class D and Class F sell altogether?

Class D	Class E	Class F
1x7	2x7
	7x2	6x2
7 u	14 u	12 u

Class D : Class E : Class F
7 : 14 : 12

Total number of mochi balls that the 3 classes had at first
= 3 x 216
= 648

Number of mochi balls that the 3 classes left
= 3 x 51 
= 153

Number of mochi balls that were sold by 3 classes
= 648 - 153
= 495

7 u + 14 u + 12 u = 495
33 u = 495

1 u = 495 ÷ 33 = 15

Total number of mochi balls sold by Class D and Class F
= 7 u + 12 u
= 19 u
= 19 x 15
= 285

Answer(s): 285