To calculate the mark-up percentage, you can use the following formula:
\[ \text{Mark-up Percentage} = \left( \frac{\text{Selling Price} - \text{Cost Price}}{\text{Cost Price}} \right) \times 100 \]
In this case, the cost price of each microwave is $84, and the selling price is $210.
Now, substitute the values into the formula:
\[ \text{Mark-up Percentage} = \left( \frac{210 - 84}{84} \right) \times 100 \]
First, calculate the difference:
\[ 210 - 84 = 126 \]
Now, substitute this back into the formula:
\[ \text{Mark-up Percentage} = \left( \frac{126}{84} \right) \times 100 \]
Now calculate the fraction:
\[ \frac{126}{84} = 1.5 \]
Finally, convert to a percentage:
\[ \text{Mark-up Percentage} = 1.5 \times 100 = 150% \]
The mark-up, as a percentage, is 150%.