[tex]7.80 = 0.1 \times 78[/tex]
So her brother delivered 78 papers, and she gave him $7.80.
Betty would get paid [tex]78 \times 0.25 = 19.50[/tex]
And from her 19.5, she gives her brother 7.80
So, at the end she has 19.50 - 7.80 = $11.70
She gets to keep $11.70 at the end of the day.