Each half-life halves the remaining count, so three half-lives leave one eighth: 64 nuclei drop to 8 while 56 have decayed. The factor 64 / 8 = 8 and the decayed total 64 - 8 = 56 are exact for the stated isotope count budget; real decay measurements have branching ratios, detector efficiency, background counts, shielding, timing uncertainty, sample purity, and finite counting statistics.
Three exact half-lives leave one eighth of the count. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.