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 

The answer options is d