Cautam nr de forma 7*k astfel incat:
50<7*k<100, unde k nr natural
Daca impartim cu 7 obtinem:
50/7<k<100/7, adica
7+1/7<k<14+2/7
si cum k nr nat, rezulta ca:
8<=k<=14, deci numerele cautate sunt: 8*7=56, 9*7=63, 10*7=70, 11*7=77, 12*7=84, 13*7=91 si 14*7=98
