Question

A store pays a 6% commission on the first $500 in sales and and 8% on sales over $500. Find the commission on an $800 sale

Answer 1

First you need to understand the structure of the commission. 
The first part is 6% commission on sales up to $500. 
The second part is 8% commission on sales over $500. 

Note that the second part doesn't mean that a sale of $800 gets 8% of $800. 

It means that once we have reached $500, any *further* sales get 8%. 
Since $800 is $300 more than $500, the commission is 

6% of $500 + 
8% of $300 

The first thing she did was work out the excess above $500. 
Then she worked out the two percentages in two separate parts. 

The second part seems to have incurred a cut and past accident, not sure if that is yours or it was already done.