Numéro 99 urgent svp merci

a-
13 x 101= 13 x 100 + 13 x 1
13 x 101= 1300 +13
13 x 101=1313

b-
32 x 99 = 32 x (100 - 1)
32 x 99 = 32 x 100 - 32 x 1
32 x 99 = 3200 -32
32 x 99 =  3168
c-

3.7 x 20.9 - 3.7 x 0.9 =   3.7 x( 20.9 - 0.9 )       
3.7 x 20.9 - 3.7 x 0.9 =   3.7 x ( 20)   
3.7 x 20.9 - 3.7 x 0.9 =   74

d-

4.28 x 3.3  + 6.7 x 4.28 = 4.28 x ( 3.3  + 6.7 )
4.28 x 3.3  + 6.7 x 4.28 = 4.28 x (10 )
4.28 x 3.3  + 6.7 x 4.28 = 42.8

astucieusement :

32*99 =                                                 
32*(100-1)
(32*100)-(32*1) =
3200-32 = 3168

3,7(20,9-0,9) = 3,7(20) = 74