Why the base rate is wrong
The FLSA regular rate is total straight-time pay for the week divided by total hours. Someone paid $20 for 30 hours and $25 for 15 hours earned $975 for 45 hours, a regular rate of $21.67. The 5 overtime hours are already paid at straight time inside that $975, so the employer owes an extra half of $21.67 for each: $54.17, not the $50 you'd get from the $20 base or the $62.50 from the $25 rate.
Bonuses count
A nondiscretionary bonus, meaning one promised for hitting a target, attendance, or production, is added to the week's pay before dividing. A $100 bonus in that same week lifts the regular rate to $23.89 and the overtime premium to $59.72. Purely discretionary gifts, like a surprise holiday bonus, are excluded. Shift differentials are included the same way; see the shift differential calculator.
Checking your stub
If your overtime line shows exactly 1.5x your lowest rate on a week where you also worked at a higher rate or earned a bonus, you're owed the difference. The Department of Labor allows back pay claims for two years, three if the underpayment was willful. Start from the overtime calculator for a single-rate week.