notam n nr nuferilor din prima saptamana
a 2-a saptamana=2xn+2
a 3-a saptamana=3x(2xn+2)-3
3x(2xn+2)-3=57
6xn+6-3=57
6xn=57+3-6
6xn=54
n=54:6
n=9 nuferi in prima saptamana
R: 9 nuferi
verificare:
9 in prima saptamana
2x9+2=20 in a 2-a saptamana
3x20-3=60-3=57 nuferi in a 3-a saptamana