An easy way to pass variables between Activities is within the Intent. To do so you use something called a bundle.
To get started, create you Intent as normal. Like so:
Intent intent = new Intent(getApplicationContext(), NewActivity.class);
The create something called a Bundle. Do so by writing:
Bundle bundle = new Bundle();
The next step is to add the variable onto the Bundle which you want to pass.
Place the name of the variable in speech marks and then add on the actual variable.
After you have added all of the variables you want to pass, add the bundle onto the Intent.
Then start the Intent as normal.
To access the variable it is fairly simple.
First Declare a Bundle. Like so:
Bundle bundle1 = getIntent().getExtras();
The next step is to declare a variable the same as the kind of which you have passed.
int theTotalLeftToSpend = bundle1.getInt("castingTotalForExpenses");