Hướng dẫn giải:

a) 4 x 78 x 5 = (4 x 5) x 78 = 20 x 78 = 1560

b) 2 x 99 x 5 = (2 x 5) x 99 = 10 x 99 = 990

c) 6 x 31 x 5 = (6 x 5) x 31 = 30 x 31 = 930

d) 8 x 63 x 5 = (8 x 5) x 63 = 40 x 63 = 2520

\(99\times36+57\times99+99-84\times99\)

\(=99\times(36+57+1-84)\)

\(=99\times10=990\)

99 x 36 + 57 x 99 - 84 x 99

= 99 x ( 36 + 57 + 1 - 84 )

= 99 x 10

= 990

2364 x 37 + 2364 x 63  = 2364 × (37 + 63)

= 2364 × 100

= 236400

25,15 x (1 + 36+63) = 25,15 x 100 = 2515

X x 39 + X x 63 - X - X = 3200 

X x (39 + 63 - 1 - 1) = 3200 

X x (102 - 2) = 3200 

X x 100 = 3200

X = 3200 : 100

X = 32 

Vậy X = 32

\(=235,5.\left(36+63+1\right)=23550\)

\(\text{34,75 x 120 - 34,75 x 19 - 34,75}\)

\(=34,75.\left(120-19-1\right)\)

\(=34,75.100\)

\(=3475\)

học tốt

\(34,75\times120-34,75\times19-34,75\)

\(=34,75\times120-34,75\times19-34,75\times1\)

\(=34,75\times\left(120-19-1\right)\)

\(=34,75\times100\)

\(=3475\)

a)0,25x5,87x40=(0,25x40)x5,87=10x5,87=58,7

b)7,48x99+7,48=7,48x(99+1)=7,48x100=748

c)98,45-41,82-35,63=98,45-(41,82+35,63)=98,45-77,45=21

57.63-47.63

=(57-47).63

=10.63

=630

63 nhân ( 57 trừ 47) =63 nhân 10 bằng 630