Current addition and unit conversion: Add 21 mA and 8000 µA, and express the result in milliamperes (mA).

Introduction / Context:
Combining quantities with different SI prefixes requires careful unit conversion. In circuit analysis, currents may be reported in microamperes or milliamperes depending on sensor ranges or measurement equipment. Correct conversion prevents order-of-magnitude errors in power and sizing calculations.

Given Data / Assumptions:

• I1 = 21 mA.
• I2 = 8000 µA.
• We must report the total in mA.

Concept / Approach:
Use micro to milli conversion: 1 mA = 1000 µA. Convert µA to mA, add, and keep consistent units. Always write a short conversion equation to avoid mental slips, especially under exam pressure.

Step-by-Step Solution:

Verification / Alternative check:
Convert everything to amperes: 21 mA = 0.021 A; 8000 µA = 0.008 A; total = 0.029 A = 29 mA. The alternate route confirms the same result.

Why Other Options Are Wrong:

• 21.8 mA: Treats 8000 µA as 0.8 mA; off by a factor of 10.
• 218 mA or 290 mA: Off by factors of 10 or 100; incorrect unit scaling.

Common Pitfalls:
Misplacing decimal points when converting between micro and milli; remember that µ → m divides by 1000, and m → µ multiplies by 1000.

Final Answer:
29 mA