pret total : nr elevi = 40
pret total : (nr elevi - 1) = 42
pret total = 40 x nr elevi
pret total = 42 x nr elevi - 42 ->pretul total fiind egal, egalam cele 2 ecuatii
40 x nr elevi = 42 x nr elevi - 42
(42 - 40) x nr elevi = 42
2 x nr elevi = 42
nr elevi = 42 : 2 = 21
pret total = 40 x 21 = 840