4a+10b+11c = 420 ... 2 formula
2 minus 1 equals 3:
A+3b =105 ............... 3 formula
Type 3 brings type 2 and type 4.
2b =11c .................. 4 formula
The value range of b is only 1 1 22 33. If b is greater than 33, A in Formula 3 is negative, which is not acceptable. So b only has the value of 1 1. 22 .33. When B is 1 1, A = 72 C =