[JohanesK]

"Naturally occuring boron is composed of only two isotopes 10|5 B and 11|5 B with atomic masses of 10.0129 and 11.00931 respectively. To account for the atomic mass of 10.811, what must be the percentage abundance of each isotope?"

With my working I'm saying x10.0129 + y11.00931 = 10.811 where x and y are the decimals expressing % e.g. 76% = x = 0.76.

Now that has 2 unknowns and I can't solve it. 

This is probably extremely basic and I must have missed the lecture. Can anybody tell me how to solve this?

The answer is 19.902% 10|5 B

[Astrokel]

Form another equation!

[JohanesK]

Form another equation!

My math brain isn't working today. I've tried but can't think of another equation and can find no similar questions in my textbook or on the internet. If you know the equation, can you tell me?
Thanks

[DrCMS]

As you are told there are only two isotopes

x+y = 1