There are multiple ways to tackle this; it's good to name a couple of them before picking the approach you will use. The approach that I would use, would be 1) repeat the question (straight forward question, so no clarification) 2) telling something like "it will be interesting to estimate this number, which can be reached with one click" (in order to break that initial ice) 3) asking for a little time to structure my solution 4) presenting my approach and take confirmation that the interviewer is fine with my approach 5) calculate & communicate meanwhile: 5.1) Gross World Product = GDP America x 4 5.2) Estimate GDP of America: 300m x GDP per capita 5.3) GDP per capita = average income/month x 12 x factor(assumption to neutralise the weight of government spending, net imports/exports) (4000$/month)*(12)*(1.25)=60.000$/person 60.000$/person*300.000.000=$18B $18B*4=$72B 6. Sanity check and comment on the possible areas that could be analysed