1) 90!/89! * 29!/30!
2) 120!/119! * 4!/5!
3) 40!/38! * 2/39
4) 50!/51! * 102!/101!

1) 90!/89! * 29!/30!

90!/89! = (89! * 90) / 89! = 90

29!/30! = 29! / 29! * 30 = 1/30

90 * 1/30 = 3

2) 120!/119! * 4!/5!

120!/119! = (199! * 120) / 119! = 120

4!/5! = 4! / (4! * 5) = 1/5

120 * 1/5 = 24

3) 40!/38! * 2/39

40!/38! = (40 * 39 * 38! ) / 38! = 1560

1560 * 2/39 = 80

4) 50!/51! * 102!/101!

50!/51! = 50!/50! * 51 = 1/51

102!/101! = (101! * 102) / 101! = 102

1/51 * 102 = 2