Answer
$8.4$
Work Step by Step
To find a percentage of a number, we convert the percent to a decimal and then multiply that decimal by the number.
One way that we can convert a percent to a decimal is by simply moving the decimal point to the left two places, and dropping the percent symbol. If we start at $21\%$, and move the decimal place two places to the left and drop the percent symbol, we end up with $0.21$.
Next, we multiply: $0.21\times40=8.4$
