Singular / plural ?

Question

Here you have:  la mitad de los alumnos fueron al viaje. Half the students went on the trip.But also you gave la mitad de los jóvenes salió de la ciudad

Answer

Hola Ba,
I imagine you are referring to the use of the verb in singular or plural in this case. 
With a quantifier + de + noun in plural, both the verb in the 3rd person singular and 3rd person plural are correct. Here are some examples:

La mitad de los habitantes se fue/se fueron a la ciudad.
Half the inhabitants left the city.
El 10 por ciento de las personas votó/votaron en contra del candidato.
10 per cent of the people/population voted against the candidate.
La mayoría de los niños fue/fueron a ese viaje.
The majority of the children went on that trip.

I hope it helped.
Saludos
Inma