Aubrey owes Ronny her share of the Internet bill & Ronny owes Aubrey $16.00 for his share of the Electric bill. They want to pay each other simultaneously. So, if Aubrey hands Ronny 10 $1 dollar bills and he hands her $4 back, how much does Aubrey owe Ronny for the Internet bill?
Ronnie should have given Aubrey $16.00 but only gave her $4.00 which means Aubrey was short $12.00 on the Internet bill. Aubrey still owes $12.00 on the Internet bill. So Ronnie says I'll give you $4.00 and we are even.
Aubrey owed a total of $10.00+$12.00=$22.00 on the Internet bill. If she paid the $22.00, Ronnie would have given her $16.00 and had $6.00 left over. That's exactly what Ronnie has left over:$10.00-$4.00=$6.00