To determine how much you can contribute to a regular and roth IRA you have to calculate your compensation:
What Is Compensation?
Generally, compensation is what you earn from
working. For a summary of what compensation does and does not include,
see Table 1-1. Compensation includes all of the items discussed next
(even if you have more than one type).Wages, salaries, etc. Wages,
salaries, tips, professional fees, bonuses, and other amounts you
receive for provid-ing personal services are compensation. The IRS
treats as compensation any amount properly shown in box 1 (Wages,
tips, other compensation) of Form W-2, Wage and Tax Statement,
provided that amount is reduced by any amount properly shown in box 11
(Nonqualified plans). Scholarship and fellowship payments are
compen-sation for IRA purposes only if shown in box 1 of Form
It a also includes commissions, self-employment income, and alimony an non-taxable combat pay.
For most people it is what i in box 1 of the W-2.
For the example in the question. If the sum of Box 1's equals $3,200 that is the maximum you can contribute to all your IRAs (regular and Roth). The funds can come from anywhere. It is not related to your net check. The money can be from savings, gifts, parents, grandparents...
The IRS doesn't care about the source of the funds, only that you don't over contribute.
Of course the calculation is more complex if the person is married, and if they have access to a retirement account.