Dear student,
Please find the solution to your problem.
Let the % of boron with mass no 10.01 be x.
Percentage of Boron of mass no 11.01 = 100 – x
[10.01x + (100 – x) 11.01]/100 = 10.81
Hence, x = 20%
Percentage of Boron with mass no. 10.01 = 20%
And for boron with mass no.11.01 = 100 – 20 = 80%
Thanks and regards,
Kushagra