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a=3+6+9+........60,b=2+5+8+......59,c=1+4+7+.....58 determinati nr d =(a+b+c):(61-55)-(a-b):20x(a-c):4

Răspuns :

a=3(1+2+...+20)
a=3×20×21÷2
a=3×10×21
a=63×10
a=630