Let he percentage existence of B10 isotope be x . Therefore percentage existence of the other isotope is (100-x)
mass of B10 isotope is 10amu
mass of B11 isotope is 11 amu
therefore
10x/100+11(100-x)/100=10.2
10x+11(100-x)=1020
1100-x=1020
x=80
therefore percentage of Boron10 isotope is 80%
and Boron 11 isotope is(100-x)=20%
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