I have 5 L at 60%. How many 700 mL bottles at 42% will that make?

Alcohol conservation again, in two steps.

Total ethanol: 5.0 × 0.60 = 3.0 L.
Final volume at 42%: 3.0 ÷ 0.42 = 7.14 L.
Bottles: 7.14 ÷ 0.70 = 10 bottles, with about 140 mL left over for the tasting glass.

Contraction shaves the real final volume slightly below 7.14 L, so plan for 10 bottles rather than hoping for 11, and remember the practical losses: racking, filtering and the wet film every vessel keeps all come out of the same budget.