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Jurubitadavid
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(245:5+5 X a ): 9+89=100

Răspuns :

ShakeStar2000
(49 + 5 × a) : 9 + 89 = 100
(49 + 5 × a) : 9 = 100 - 89
(49 + 5 × a) : 9 = 11
49 + 5 × a = 11 × 9
49 + 5 × a = 99
5 × a = 99 - 49
5 × a = 50
a = 50 : 5 
a = 10
Coitannyna
(49 + 5 × a) : 9 + 89 = 100
(49 + 5 × a) : 9 = 100 - 89
(49 + 5 × a) : 9 = 11
49 + 5 × a = 11 × 9
49 + 5 × a = 99
5 × a = 99 - 49
5 × a = 50
a = 50 : 5 
a = 10
rs:10