You have 20 data values.
Multiply 0.60 * 20= 12. 12 is your index.
The 60th percentile is the average of the 12th and 13th value. (21 + 34)/2 = 27.5
You did the average because 12 was an integer value. If the multiplication had given you (for example) 11.2, you would have rounded up to 12 and the 60th percentile would have been 21.
There are other schemes for calculating percentiles. Don't panic if your answer differed a small amount from someone else's. Just double check your arithmetic.
I hope this helps!