Atomic weight of boron is 10.81 and it has two isotopes 105B and 115B in nature would be :
The nature would be given as
AtomicWeight=(abundance×atomicmass+abundance×atomicmass)100
Letx be the abundance of the105B
(100−x)be abundance of115B
So on solving the above equation
10.81=10x+(100−x)11100
x=19
We get the abundance ratio is1981