Realise that this is a weighted average or mixture problem.
10*15
7*6
8*x
9 (21+x)
150+42+8x=9x+189
x=3
3:21
1:7
10*15
7*6
8*x
9 (21+x)
150+42+8x=9x+189
x=3
3:21
1:7
