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{[(3×a-1)÷7+1]×2+4}×2=20

Răspuns :

[(3×a-1)÷7+1]×2+4=20:2

[(3×a-1)÷7+1]×2+4=10

[(3×a-1)÷7+1]×2=10-4

[(3×a-1)÷7+1]×2=6

(3×a-1)÷7+1=6:2

(3×a-1)÷7+1=3

(3×a-1)÷7=3-1

(3×a-1)÷7=2

3×a-1=2×7

3×a-1=14

3×a=14+1

3×a=15

a=15:3

a=5