Răspuns:
[(99 - a ×3) :9 +89] :10 =9
[(99 - a ×3) :9 +89] =9 ×10
[(99 - a ×3) :9 +89] =90
(99 - a ×3) :9 =90 - 89
(99 - a ×3) :9 =1
99 - a ×3 =1×9
99 - a ×3 =9
a ×3 =99 - 9
a ×3 =90
a =90 :3
a =30
Verificare
[(99 - 30 ×3) :9 +89] :10 =9
[(99 - 90) :9 +89] :10 =9
(9 :9 +89) :10 =9
(1 +89) :10 =9
90 :10 =9
9 =9