We have that we can represent the total sales with the following expression:

Now, we have that the sales person earns 5% comission on the first $5000, this is equivalent to:

then, if the salesperson earns $1375 in comissions, we have the following equations:

we have that the sales with 7.5% comissions got the salesperson $1125, then, we have the following:

therefore, the salesperson had 15000+5000=$20000 in sales to earn $1375 in comissions