... by: Dieter

#673 - initially populated cells: 24 = 29,6%
No solver used, solver = brain.
basics: D7=4, E8=6, F9=7
populated cells: 27 = 33,3%
Strategy: Numbers 7,4,9,2,6 occur most frequently, combine two cells.
combinations of A4 = (4,7,9) and F1 = (2,5,6)
2 matrices left after basic checks: A4/F1 = 7/2 and 9/5
A4/F1 = 7/2 solves with F4=3

Optimization:
F1=2 & F4=3 solve.