Complete the square in the denominator.

Rewrite the given transform as

Now take the inverse transform:

Setup
0.8x + 0.4y = 50
0.1x + 0.2y = 10 - Multiply by -2 and add
get
0.6x = 30
x = 50 mg compound A
.1 (50) + .2y = 10
5 + .2y = 10
.2y = 5
y = 25 mg compound B
A) (1 x 4) + (1 x 4) = 8
There are 8 possible combinations
B) RH:RD:RC:RS:BH:BD:BC:BS
Hope this helps and have a nice day!
I’m pretty sure the answer is 12