When two numbers in scientific notation have different powers of 10, you must perform an important step before adding and subtracting them: you must first establish a common power of 10 for both numbers. If you want to add 1.20×10¹ and 2.40×10², you cannot add 1.20 and 2.40 to yield 3.60. Why? Because the two numbers you are adding are 12.0 and 240, which sum to 252, not 36 or 360.

In this example, the numbers in scientific notation have different powers of 10: the first has 10 to the first power, the second, 10 squared. This means you must first convert to a common power of 10. For example, convert the first number to 0.120×10² and then add to obtain 2.52×10².