Radius-cubed and period-square scale together in this normalized law. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
The larger orbit has eight times radius-cubed
The radius-cubed grows from 8 to 64, a factor of 8.
rA3rB3=864=8
The period-square grows by the same factor
The period-square grows from 4 to 32, also a factor of 8.