Ninth, I would normally round out numbers to make the mental arithmetic easier and chose the multi guess answer that was nearest. This has worked out well in tests over the years. On one multi guess the answer I came up with was one of the options, however luckily I noticed that another answer was close as well, and doing the maths properly I got the correct answer. (The difference was 2.5V so my suspicions were raised on that occasion). Even today although mains voltage in the UK is 230V, I still use either 240 or 250 as they are easier numbers to do mental arithmetic with.<br /><br />
And remember, if in doubt...answer "C". Amazing how many times that one was true.