Question

afla numerele necunoscute a+9=21 c+c=12 15+c=30 b-13=17 b-5=18 30-c=19 12-c=3

Answer 1

   a+9=21  =>  a = 21-9 = 12 ;
   c+c=12   =>  c = 12:2 =  6 ;
15+c= 30   =>  c = 30-15 = 15 ;
b - 13 = 17 =>  b =17+13 = 30 ;
b - 5 = 18   =>  b = 18+5  = 23 ;
30 - c = 19 => c = 30-19 = 11 ;
12 - c = 3   => c = 12 - 3 = 9 ;