Jerry bought a package of pens contain a 20*3/4 of the pens had black ink how do you determine the number of pants with black ink

If you're saying there were 20 pens and 3/4 of them had black ink, you divide 20 by 4, the denominator, to get 5. Once you have the number 5 you multiply it by the numerator, 3, to get 15. 15 of the pens had black ink.