Costi ______/ 4
Cosmin ______/______/___/......................total=28
Costi=( 28-4 ) : 3 =8 ani
Cosmin= 2x 8 +4= 20 ani sau = 28-8=20 ani
8..........20-acum
7.........19
6..........18
5...........17
4............16....4x4=16..............in urma cu 4 ani
sau algebric
(8-a) x 4= 20-a
32-4a=20-a
3a= 32-20
3a=12
a=4 ani