The total number of nucleotides in 1 gen is 1500.
The percentage of cytosine nucleotides is 20%.

To find the number of cytosine nucleotides, we can multiply the total number of nucleotides by the percentage of cytosine nucleotides:
Number of cytosine nucleotides = 1500 * 0.20 = 300

Since there are four types of nucleotides (adenine, thymine, cytosine, and guanine), we can divide the remaining nucleotides equally among the three types (adenine, thymine, and guanine):
Number of nucleotides for each type (adenine, thymine, guanine) = (1500 - 300) / 3 = 400

Therefore, in 1 gen, there are 300 cytosine nucleotides and 400 nucleotides for each of the other three types (adenine, thymine, and guanine).