Electron-family and muon-family ledgers must close separately. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Electron family cancels in a pair
Electron-family total 0 to 0; muon-family total 0 to 0; accepted bit 1.
0→0,0→0,a=1
Muon decay keeps two family ledgers
Electron-family total 0 to 0; muon-family total 1 to 1; accepted bit 1.
0→0,1→1,a=1
A missing neutrino leaves both families wrong
Electron-family total 0 to 1; muon-family total 1 to 0; accepted bit 0.