`1.`
`a) 21 . 4`
`= 21 . 2 . 2`
`= 42 . 2`
`= 84`
`b)`
`44 . 25`
`= 11 . 4 . 25`
`= 11 . 100`
`= 1100`
`c)`
`125 . 56`
`= 125 . 8 . 7`
`= 1000 . 7`
`= 7000`
`d)`
`19 . 8`
`= 19 . 2 . 4`
`= 38 . 4`
`= 152`
`2.`
`a) 91.11`
`= 91 . (10 + 1)`
`= 91 . 10 + 91`
`= 910 + 91`
`= 1001`
`b) 45 . 12`
`= 45 . (10 + 2)`
`= 45 . 10 + 45.2`
`= 450 + 90`
`= 540`
`3.`
`a) 45 . 29 = 45 . (30 - 1) = 45 . 30 - 45 . 1 = 1350 - 45 = 1305`
`b) 47 . 98 = 47 . (100 - 2) = 47 . 100 - 47 . 2 = 4700 - 94 = 4606`
`c) 15 . 998 = 15 . (1000 - 2) = 15 . 1000 - 15 . 2 = 15000 - 30 = 14970`
`4`
`a) 5 . 11 . 18 + 9 . 31 . 10 + 4 . 29 . 45`
`= 18 . 5 . 11 + 9 . 310 + 45 . 29 . 4`
`= 9 . 2 . 5 . 11 + 9 . 310 + 9 . 5 . 29 . 4`
`= 9 . 110 + 9 . 310 + 9 . 580`
`= 9 . (110 + 310 + 580)`
`= 9 . 1000`
`= 9000`
`b) 37 . 39 + 78 . 14 + 13 . 85 + 52 . 55`
`= 37 . 39 + 39 . 2 . 14 + 13 . 5 . 17 + 13 . 4 . 5 . 11`
`= 39 . (37 + 28) + 65 . 17 + 65 . 44`
`= 39 . 65 + 65 . 17 + 65 . 44`
`= 65 . ( 39 + 17 + 44)`
`= 65 . 100`
`= 6500`