Calculating percentages based on a known ratio but unknown values

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I am mixing a chemical solution and working up a chart for this.

The ratio is:

49% part 1
49% part 2
2% part 3

Part 1 and part 2 are always the same quantity.

If I start from a known total quantity — ie, “I want to make a total of 1000mL of solution” — this is easy to calculate:

Total x mL = (x mL * .49) + (x mL * .49) + (x mL *.02)

My question is this: is there a way to calculate the amounts “backwards”.

Example: “I have 5mL of part three. How many mL of part 1 and part 2 do I need?”

Or: “I have 8 mL of parts 1 and parts 2. How many mL of part 3 do I need?”

(Edited to clarify question)

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Yes, if I understood you correctly. If everything is in mL. If you have 5 of part 3, then the total will be $5/0.02=250$, and from that you can calculate the other parts.