One half-life leaves exactly half the starting nuclei, so a population of 64 drops to 32 remaining while the other 32 have decayed. The halving 64 / 2 = 32 and the decayed total 64 - 32 = 32 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.
One exact half-life leaves half the starting nuclei. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.