there is another trick to solve these queries.
Always talk with respect to Unit currency.
In the above question the US firm has Euro and he needs US $ so as per qoute unit currency is $ so lets talk Wrt $ ...$ requires to be purchassed and as we know it cna be purchased at market selling rate i.e ask rate so relevant rate is 0.800 per $
so final the us firm get = 120000/0.800 =150000$ ans.