20. Scrieţi 3 exemple de numere naturale care au exact trei divizori.
21. Scrieţi 5 exemple de numere naturale care au exact doi divizori.
22. Să se afle suma divizorilor lui : a) 9; b) 12; c) 33.
23. Să se afle suma multiplilor lui 10 cuprinsi între 23 si 53.
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