The formula is
1/f=1/v+1/u
f=focus posiy
v=image position
u=object position
Now here we have
object at 30 cm hence u=-30
We have our focal length 30 cm.
Hence f=30
Now 1/30 =1/v-1/30
Or,2/30=1/v
Or,1/15=1/v
Or,v=15.
Hence we can say that the image is at 15 cm from the centre