How do you calculate the percentage abundance of the two isotopes of rubidium 85R and 87R?

Can someone please help me do this? Im a little stumped on this question. Please and Thank youHow do you calculate the percentage abundance of the two isotopes of rubidium 85R and 87R?
The atomic mass of Rubidium is 85.47



(some decimal) x 85 + (some decimal) x 87 = 85.47



85x + 87y = 85.47



Since x and y are both decimals that add together to give 1 (100% of rubidium is either Rb-85 or Rb-87);



x+y=1



This means that;



x=1-y



Substitute.



85(1-y) + 87y = 85.47



Simplify.



85-85y+87y= 85.47

2y= 0.47

y= 0.235



x+0.235=1

x=0.765



This means that there is 76.5% Rb-85 and 23.5% Rb-87 in nature.