Lets say I have this:

```
1A=6 2A=4.5  3A=6

1B=7 2B=6    3B=7.5

1C=6 2C=6.75 3C=9

```

I want to find the combination of numbers that yields the highest sum. None of the numbers before the letters can be used more than once and none of the letters after the numbers can be used more than once. eg. `1A+2B+3C, 1C+2B+3A` are valid. `1A+1B+2B,3A+2B+3B,1A+2A+3A` are invalid.

In this case the highest sum is `1A+2B+3C=21`. How would I find this result and combination using python?

Thanks in advance