The greatest common factor (GCF) of 15 and 26 is 1.
To find the GCF, we start by identifying the factors of each number. The factors of 15 are 1, 3, 5, and 15. The factors of 26 are 1, 2, 13, and 26. Next, we look for the largest factor that both numbers share. In this case, the only common factor between 15 and 26 is 1. Therefore, since there are no larger factors that both numbers share, the GCF is 1.