a/b=14/15=>a=14b/15
b/c=5/7=>c=7b/5
a/c=14b/15/7b.5=14b/15: 7b/5=14b/15 x5/7b (pe digonala 14b cu 7 b se simplifica si in loc de 14b vom avea 2 iar in loc de 7b vom avea 1=2/15 x5/1 (pe diagonala 5 cu 15 se simplifica si in loc de 5 avem 1 iar in loc de 15 avem 3)=2/3x1/1=2/3
deci a/c=2/3
