If you put 1/2 of 36 mg and 1/2 of 11 mg, then you add 1x36 and 1x11 (= 47); then divide it by the bottom number, so 2. Result: 23.5
If you would put 1/3 of 36 mg together with 2/3 of 11 mg, you would have to add 1x36 and 2x11 (so the top numbers); result here 36+11+11=58 and divide that by 3 (bottom number, that needs to be the same for all parts you add). Result: 58/3 = 19,3
If you would put 1/3 of 36 mg, and divide what's left into 1/4 of 11 mg and 3/4 of 6 mg, you first need to find a number at the bottom, into which you can 'translate' all the figures you have so far... just start counting at 1, and for each number you reach, you look if you can divide that number with the bottom numbers you have, reaching a whole number.
In this case, it's bingo at 12: you can divide that by 3 and by 4 (you are working with thirds and fourths here). Now translate everything you had into this 12-number at the bottom.
Meaning: 1/3 is the same as 4/12th... leaves 8/12ths that you divide between the other two... 2/12ths for the 11 mg, 6/12ths for the 6 mg (if you add the top numbers you can see you did the right math so far: 4+2+6=12, so 12/12th = 1, the whole package you are dealing with). Now the rest of the sum... you add 4X36mg + 2x11mg + 6x6mg (=202) and divide that by the bottom number, 12. Result: 202/12=101/6=16,85mg
Have fun making your recipee's
(Of course it's easier using a calculator and percentages; but this is a way you can do it 'in your head' or on a scrap of paper, just following the rules set out above)