(720-715)×a:(9+9:9×21)+398=400​

Răspuns :

5 × a : (9 + 21) + 398 = 400

5 × a : 30 + 398 = 400

5 × a : 30 = 400 - 398

5 × a : 30 = 2

5 × a = 2 × 30

5 × a= 60

a= 60 : 5

a= 12