I am a Computer Science student, preparing for my exam in software engineering.

I am strugging with answering one of the sample questions to the scenario below. My understanding is that the system design approach should probably be a mixture of agile and plan driven elements but - since I've no practical experience - it's hard for me to decide on the balance and tolls that should be used.

I will appreciate any hints from experienced business analysts who were involved in similar kind of projects.

Ray Sing is the owner of “Last Betz", a bookmakers with 7 outlets across Louth and Meath. With the advent of smartphones Ray would now like to allow his clients to place their bets online using their mobile devices. Clients would register for an account and password and would log their credit card details via the Last Betz website. To begin using the facility customers must 'load' their accounts with 100 euros. Any winnings, minus commission, will be placed in the account whilst any losses will be automatically deducted from the account. Assuming you have been selected to develop the above system:

How would you approach the design of this system? Discuss the design methods and models you would use.