20+30+10 = 60-40+80 = 50+10+10 = 20+30+10 = 60-40+80 = 50+10+40 = 20-10+80 = 30+60-90 = 70-20+40=

Будем решать по действиям, начиная считать слева на право.

а) 20 + 30 + 10 = 60:

1. 20 + 30 = 50;

2. 50 + 10 = 60.

б) 60 - 40 + 80 = 100

1. 60 - 40 = 20;

2. 20 + 80 = 100.

в) 50 + 10 + 10 = 70:

1. 50 + 10 = 60;

2. 60 + 10 = 70.

г) 20 + 30 + 10 = 60:

1. 20 + 30 = 50;

2. 50 + 10 = 60.

д) 60 - 40 + 80 = 100.

1. 60 - 40 = 20;

2. 20 + 80 = 100.

е) 50 + 10 + 40 = 100:

1. 50 + 10 = 60;

2. 60 + 40 = 100.

ж) 20 - 10 + 80 = 90:

1. 20 - 10 = 10;

2. 10 + 80 = 90.

з) 30 + 60 - 90 = 0:

1. 30 + 60 = 90;

2. 90 - 90 = 0.

и) 70 - 20 + 40 = 90:

1. 70 - 20 = 50;

2. 50 + 40 = 90.