What is 40% of 24?

A “percent” (%) is just what is says - “per cent”, or per one hundred. So, 40% is $\left(\frac{40}{100}\right)$, or 0.40. Multiplying this value by the “total” we want – 24 in this case – gives us 9.6.
This can be checked by taking the inverse – dividing 24 by 9.6: $\left(\frac{9.6}{24}\right)$ = 0.40. That is a unit ratio, or parts per “1” (one). Multiply that by 100 to get “parts per hundred” or “percent” 0.40 * 100 = 40.