(100a+10b+c): (10a+b+10a+c)=5 rest 5
100a+10b+c= 5(20a+b+c) +5
100a+10b+c=100a+5b+5c+5
10b-5b=5c-c+5
5b=4c+5
5b-5=4c
5(b-1)=4c
b-1=4, b=5
c=5
a ∈ {1,2,3,4,5,6,7,8,9} 9 valori
b=5 - 1 valoare
c=5 - 1 valoare
nr de numere care respecta cerinta e 9*1*1=9
