If the bearing of a from be is 100⁰ what is the bearing of b from a

The bearing of A from B is $100^\circ$.

If the given bearing is less than $180^\circ$, add $180^\circ$ to find the back bearing.
If the given bearing is greater than $180^\circ$, subtract $180^\circ$ to find the back bearing.
Since $100^\circ$ is less than $180^\circ$, we add $180^\circ$.

Bearing of B from A = Bearing of A from B + $180^\circ$
Bearing of B from A = $100^\circ + 180^\circ$
Bearing of B from A = $280^\circ$

The bearing of B from A is \boxed{\text{280^\circ}}.