It is 41 bases (or nucleotides) long. In this example, the flanking sequences consist of 9 bases (although in reality flanking regions are typically much longer). The eight four-base repeat units (GATA) make up the remaining 32 bases. How long would you expect an allele from this same locus to be if it had 10 repeat units

49

It says that there are eight four-base repeat unites that make up the remaining 32 bases. This means that it 8 x 4 = 32. Then from 32 + 9 = 41 is how you get all of the bases in the original. However, if there were to be 10 repeat units then it would be 10 x 4 = 40. Then from there you would get 40 + 9 = 49. Hope my explanation helped somewhat.