Proportional percentage increases in groups

Question

If I have 4 gases in an atmosphere, lets say:

• A makes up 78%
• B makes up 21%
• C makes up .93%
• D makes up .04%

If the percentage of gas D increases to 5%, how do I find the proportional percentage decrease in the others?

*The actual question is if given that the others decrease proportionally, is if the atmosphere is molecularly heavier

A being Nitrogen, B being Oxygen, C being Argon, D being CO2

Answer 1

From 0.04% to 5.00% we added 4.96%.

So the other components must be decreased proportionally by (99.96-4.96)/99.96 = 95.00/99.96.

Where 99.96 = 100 - 0.04.

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A - NITROGEN

INITIAL = 78%

FINAL = 78% (99.96-4.96)/99.96 = 74.129652% 

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B - OXYGEN

INITIAL = 21%

FINAL = 21% (99.96-4.96)/99.96 = 19.957983%

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C - ARGON

INITIAL = 0.93%

FINAL = 0.93% (99.96-4.96)/99.96 = 0.883854%

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D - CO2

INITIAL = 0.04%

FINAL = 5.00000 % (Increase 4.96%)

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E - OTHERS

INITIAL = 0.03%

FINAL = 0.03% (99.96-4.96)/99.96 = 0.028511%

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