Deliveroo Software Engineer interview questions

In the architectural interview, I was asked how I would build a system to give out 100k voucher codes for free burgers, with some constraints like: - they expect all to be redeemed within a few minutes - we couldn't give out more than one voucher to the same person (I forget more) They wanted to know what system I would design to handle the volume of traffic efficiently, how the users could be segmented to different servers and things like that. It was a great question, which I apparently didn't give an adequate answer to, but I would really love to know what a great answer would be