Two orbit ledgers match the same normalized Kepler ratio. 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 first orbit matches the normalized period law
The first period-square 4 over radius-cubed 8 gives one half.
rA3TA2=84=21
The second orbit matches the same normalized law
The second period-square 32 over radius-cubed 64 also gives one half.