Bromine has two isotopes, Br- 79 and Br- 81. The relative atomic mass of bromine is 79.9.

CALCULATE THE PERCENTAGE of Br-79 atoms in a sample of bromine.

PLease help i'm really stuck

2 answers

The total for 79 and 81 is 100% and the total for fraction is 1, so
let X = fraction 79 and 1-X = fraction of 81.
Then 79(X) + 81(1-X) = 79.9
Solve for X.
I dont get it the total is 160 and there are2 numbers so 2 fractions? and I dont get the x thing with 1-x AND 79(X)+81(1-X) iM VERY CONFUSED PLEASE HELP