Boron has only to naturally occurring isotopes. The mass of boron-10 is 10.01294 amu and the mass of bornon-11 is 11.00931 amu. Calculate the relative abundances of the two isotopes

X=percent as a fraction of boron 10

y=percent as a fraction of boron 11

The percentages as fractions add to equal one

x+y=1

x=1-y

10.01294x+11.00931y=10.811

substitute in the value of x from equation 1 in to equation 2

10.01294 (1-y) + 11.00931y=10.811

multiply out and solve for y

10.01294-10.01294y+11.00931y=10.811

0.99637y=0.79806

y=0.800967512 = approx 80.1 % boron 11 isotope

100%-80.1%=19.9% boron 10 isotope