rides calculator app (android)

[lindomar]

I need a simple app to do the following:
I take rides with a colleague, and I want an app to show me how much I owe him. It would basically be like

Rides
"value" +
//the plus sign is a button to increment a ride (and adds +3 for debit)

Total paid
"value"
//how much I already gave him, value is edited manually

Debit
"value"
//each ride is 3 bucks, so debit=(rides*3)-(total paid)


THANKS A LOT