The abundance of 12c is 98.9%. 13c is another naturally occurring isotope. what is the percent abundance of 13c?

Actually the total abundance of the isotopes of any element in this world must sum up to 100%. So we initially know that 12 C is 98.9 percent abundant, therefore the remaining of the 100 percent must be of 13 C, that is:

13 C = 100% - 98.9%

13 C = 1.1%