I am practicing mathematical approaches.
I got all good, except this one:
The excess of $a$ over $b$ is $n$ units
I think that excess mean $+$, so $b + a$ = n, because is $a$ over $b$ is equal to $n$.
Although in this case, it does not matter, since it is a sum.
So, my result was:
$b + a = n$
I also thought of it as:
"The units that make $ a $ greater than $ b $ are $ n $", then:
a + n = b
But the correct result is: $a - b = n$
But, this is not close either, since this result is: $ a - b = -n $.
I think my mistake is in the meaning of "exceeds"
"The excess of $a$ over $b$ is $n$ units" means "$a$ is greater than $b$ by $n$ units", so $a=b+n$.