3,999,960. 120 possible combos, 24 per number, i.e. 1 has 24 combos, 2 has 24 combos, etc. 15 x 24 = 360 (15 bc you add 1,2,3,4,5) ones digit - 360 tens digit - 360 x 10 = 3,600 hundreds - 3,600 x 10 = 36,000 thousands - 36,000 x 10 = 360,000 ten-thousands - 360,000 x 10 = 3,600,000 Add them all together for 3,999,960.
