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
