Answer : The correct answer is:
The abundance of isotopes can be found out using following formula :
atomic mass of element = ( abundance of isotope 1 * mass of isotope 1 ) + ( abundance of isotope 2 * mass of isotope 2 ) +.....
Boron ( mass = 10.811 u ) has two isotopes : ¹⁰B ( mass = 10.0129 u ) and ¹¹ B ( mass = 11.0093 u )
Sum of abundance of all isotopes = 100%
Lets assume abundance of isotope ¹¹B = x % = 0.01 x , so abundance of isotope ¹⁰B = 100 % - x% = 100/100 - x/100 = 1 - 0.01 x
Plugging values in formula :
10.811 = ( 0.01 x * 11.0093) + [( 1 - 0.01 x) * 10.0129 ]
10.811 = (0.110093 x ) + ( 10.0129 - 0.100129 x )
10.811 - 10.0129 = 0.110093 x - 0.100129 x
0.7981 = 0.009964 x
Dividing both side by 0.00964
0.7981 / 0.009964 = 0.00964 /0.00964 x
x = 82.7 %
Abundance of of isotope ¹¹ B = 82.7 %
Abundance of isotope ¹⁰ B = 100% - x % = 100 %- 82.7 % = 17.3 %