Maths Question, in this example there are 3 fertz, can you balance the NPK

you can use ppm or weighted average percentages.. both can lead to the same end goal and require experience/familiarity over time of use. total amount of each products dosing.. say it;s 3mL, 5mL and 6m = 14 total mL per gallon or Liter etc. so (3/14) x each of the first products NPK values et al. (5/14) x for the 2nd, and (5/14) x 3rd ther are apps that will calculate dose based on target PPMs. i'm sure some can do weighted average of percentages too. growbuddy? hydrobuddy? one of those 2 can do it. I've found something similar on web too. If you want to calculate elemental ppm, just google for the equation and make a spreadsheet. but, others have already done this work for you. With nitrigen sources that require microbes and such to break them down, i believe you want a 2-1-2 ratio - exactly why you see a 20-10-20 option at the store.. Realize these sort of generalizations are meant as a ballpark idea and you may need to adjust from there a bit. With soilless nutes you go for a 1-1-2 becaue the nitrogen is 100% plant available when dissolved. K-Ca-Mg is usualy suggested at 4-2-1 ratios but with marijuana, you definitely want more Mg than that, in my experience.