Muon decay keeps electron-family and muon-family ledgers separate. 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 number cancels
The electron-family output is 1 plus -1 plus 0, giving 0.
1+−1+0=0
Muon-family number remains one
The muon-family input is 1 and the output is 0 plus 0 plus 1.