etan = C2H6 , avem a moli , M= 30 g/mol
alchena = CnH2n , avem b moli , M= 14n g/mol
a + b ............................ 100% amestec
a .................................. 75%
a+b = 1,333a => b = 0,333a
(2a+nb)/(6a+2nb) = 3/8
=> 18a + 6nb = 16a + 8nb => 2a = 2nb => a = nb
n = a/b = a/0,333a = n = 3 => C3H6, propena