I think for the duration of the bang, as 1 Watt = 1 Joule per second, you had to use the 5% of the 500J thing to give you 25J, then I think you did 25J/200W which gives you 125ms.
Ridiculous paper though. A shame that you spend hours and hours of your time revising heavily for one exam, then they give you a dodgy paper that half of it isn't even related to the syllabus. It was heavily maths based, unorganised and all over the place.