I did it in a different way. So what you would do, would be to create a table, with 10B and 11B at the top. Under those, write x, and 100-x. The reason for writing x is because thats the one we need to find out, but for 100-x it's because both of the values for "x" have to equal 100.
Seeing as the RAM is 10.8, you would work backwards.
10.8 = 10 (x) + 11 (100-x) /100. You would multiply the values of the Borons together with the x to get the unknown. Divide it by 100 to get the percentage.
After that, it's basic solving. I got 1080 = 10x + 1100 - 11x. (got the 1080 by multiplying it by 100). 10x-11x is -x.
1080=-x+1110
1080 - 1100 = -20
-20 = -x, therefore, x is equals to 20