First find the no. of moles of NaOH :
30/1000 = 0.3 dm3 so no. of moles = 0.3*0.5 = 0.15 moles
as NaOH reacts with HNO3 in a ratio of one to one, there must have been 0.15 moles of HNO3 too
moles/volume = concentration
volume= 15/1000 = 0.15 dm3
concentration = 1.15/0.15 = 1 mol.dm-3
The quicker way would be to realize that you used twice as much NaOH so the HNO3 had to be twice as strong
