Solution
For this case we can use the following equations:
A+ B= 27
40 A + 30 B = 930
If we solve A for the first equation we got:
A= 27 -B
And replacing in the second we got:
40 (27-B) + 30 B= 930
1080 -40B + 30B = 930
-10B= -150
B= 150/10 = 15
And slving for A we got:
A= 27-15= 12