n = 12·cat₁ + 6 = 15· cat₂ + 9 = 18· cat₃ + 12 adunam 6
n+ 6 =12 ·cat₁ + 12 = 15· cat₂ + 15 = 18·cat₃ + 18
n + 6 = 12· ( cat₁ + 1) = 15· ( cat₂ + 1) = 18· ( cat₃ + 1)
n + 6 este multiplu de : 12 = 2² · 3
15 = 3 · 5
18 = 2 ·3²
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multiplu = 2² · 3² · 5 = 4· 9 ·5 = 180
n + 6 = 180
n= 174