Is the difference significant.

#1
I'm using stata since only a few weeks for my thesis. I can use some help regarding the following:
I wanted to know what the mean grade is for different categories of hours of work (tijdwerku). Therefore I did the following, as this last variable was not yet divided in categories:
sum tentcijf if xjaars==1 & tijdwerku ==0
sum tentcijf if xjaars==1 & tijdwerku >0 & tijdwerku <=5
sum tentcijf if xjaars==1 & tijdwerku >=6 & tijdwerku <=10
sum tentcijf if xjaars==1 & tijdwerku >=11 & tijdwerku <= 15
sum tentcijf if xjaars==1 & tijdwerku >=16 & tijdwerku <= 20
sum tentcijf if xjaars==1 & tijdwerku >=21
This gave me different means of grade (tentcijf).
However, now I want to see/know whether these means are significant different from each other. It makes it difficult that tijdwerku (hours work) has no subgroups itself I think? Does anyone know how I can find out?
Many thanks in advande