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The of boron is 10.81 u.
And 10.81 u is a lot closer to 11u than it is to 10u, so there must be more of boron-11.
To convince you fully, we can also do a simple calculation to find the exact proportion of boron-11 using the following formula:

$((10 u)(x)+(11 u)(1-x))/(100%)=10.81u$

Where u is the unit for atomic mass and x is the proportion of boron-10 out of the total boron abundance which is 100%.

Solving for x we get:

$11u-ux=10.81u$
$0.19u=ux$
$x=0.19$
$1-x=0.81$

And thus the abundance of boron-11 is roughly 81%.

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