(1)12.4-2.4÷
4
15 ÷2.5,
=12.4-9÷2.5,
=12.4-3.6,
=8.8;
(2)(
4
5 +
1
4 )÷
7
3 +
7
10 ,
=
21
20 ×
3
7 +
7
10 ,
=
9
20 +
7
10 ,
=
23
20 ;
(3)99999×17+33333×49,
=33333×51+33333×49,
=33333×(51+49),
=33333×100,
=3333300;
(4)
1
5×8 +
1
8×11 +
1
11×14 +
1
14×17 +
1
17×20 ,
=
1
3 ×(
1
5 -
1
8 )+
1
3 ×(
1
8 -
1
11 )+
1
3 ×(
1
11 -
1
14 )+
1
3 ×(
1
14 -
1
17 )+
1
3 ×(
1
17 -
1
20 ),
=
1
3 ×(
1
5 -
1
8 +
1
8 -
1
11 +
1
11 -
1
14 +
1
14 -
1
17 +
1
17 -
1
20 ),
=
1
3 ×(
1
5 -
1
20 ),
=
1
3 ×
3
20 ,
=
1
20 .