Calculate weekly net pay
I'd start extracting some more functions.

What about decomposing

int NetPay(int hours){
    int payedBeforeTaxes = payedBeforeTaxes(hours);
    return applyTaxes(payedBeforeTaxes);
}

int payedBeforeTaxes(hours){
    return regularPay(hours) + overtimePay(hours);
}

int applyTaxes(int amountBeforeTaxes){
    int firstRateTaxes  = applyFirstRate(amountBeforeTaxes);
    int secondRateTaxes = applySecondRate(amountBeforeTaxes);
    int thirdRateTaxes  = applyThirdRate(amountBeforeTaxes);
    return amountBeforeTaxes - firstRateTaxes -
           secondRateTaxes - thirdRateTaxes;
}


At this point you just need to implement the functions with the appropriate code.
You'll notice that with this structure each function only does one thing and it will be much simpler and clearer.

Alternatively you can replace your apply taxes introducing an array of `(rate, threshold).
You will have to loop through the array, and at each step compute the amount you have to apply that tax rate and the associated tax amount.