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2014-07-07T16:27:17+03:00
D:I=C, +R
D=I *C

D*I*C=441
I*C*I*C=441

IC²=441   ⇒IC=21   ⇔D=21

21:I=C

I,C∈{3;7}

I=3
21:3=7   ⇒C=7

I=7
21:7=3   ⇒C=3