Hii

Conversion of CGPA to percentage is done by multiplying total CGPA into 9.5 so reciprocal of this will help you in converting percentage into CGPA.
290 marks out of 500 means you have total 58%.
58% divided by  9.5 will give you your cpga which is 58 / 9.5 = 6.10 CGPA.
So you CPGA would be 6.10

Hope this helps
Thanks!!