What is your experience in ba maths + eco . Is a good option what is benefits

You can do a MSc for mathematics after a BA of economics. I assume your interested in statistics? Even in statistics you'll need to know how to write proofs, so whatever school you go to will want you to take the undergraduate level mathematics proofs class before getting you into the graduate level. The aim is to keep you in the program and get you a master's degree/PhD degree. To do that they want to make sure you have the recommended classes before actually having you start the graduate courses. They don't force you to take those undergraduate classes, but it's recommended if your BA programs didn't include any mathematical proof writing courses.

Best of luck. Thank you.