Number of notes=90
Let the number of Rs.50 notes be x
Then , number of Rs.10 notes=(90-x)
Value of all the Rs.50 notes=Rs.50*x=Rs.50x
Value of all the Rs.10 notes=Rs.10*(90-x)=Rs.(900-10x)
Since the number of notes is not a natural number , the question is impossible.
(Important note:You can see that if all the notes are of Rs.10 then, total value of them=10*90=Rs.900 which is <500.SO THE QUESTION IS IMPOSSIBLE)