Answer
0.00021 = 2.1 $\times$ $10^{-4}$
Work Step by Step
We need to write 0.00021 as the product of a number greater than or equal to 1 and less than 10 and an integer power of 10.
The number greater than or equal to 1 and less than 10 will be 2.1
The decimal point has moved 4 places to the left, so the exponent of 10 will be -4.
So, 0.00021 = 2.1 $\times$ $10^{-4}$