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(a×10-10×2):2=5, ...............

Răspuns :

(ax10-10x2):2=5
ax10-20=5x2
ax10-20=10
ax10=10+20
ax10=30
a=30:10
a=3
verificare
(3x10-10x2):2=5
(30-20):2=5
10:2=5
5=5
(a x 10 - 10 x 2) :2 =5 
(10a - 20 ) : 2 = 5
5a-10 = 5
5a= 5+10
5a=15
a= 15:5
a= 3