Probably the easiest way to understand is to make 2 ratios, then cross-multiply.
20 = 100% of 20, so
6/x = 20/100
Then cross multiply
600 = 20x
x = 30 percent
——————
You can also divide the part by the whole:
6/20 = 0.3, then multiply that by 100 to get the percent. 0.3 x 100= 30 percent.
Use which ever one makes the most sense to your brain.
Let me know if you have questions about this process, and I’ll happily help.