Useful mental maths

Hi
I’d like to make a webapp quiz about mental maths similar to this , but I want to focus on math operations that are useful in real life

Some of the questions I think are useful in real situations are the following:

  • Adding or substracting percentages (useful when checking discounts or invoices)
  • Unit conversions (useful when comparing prices of different packages)
  • Date/time arithmetics (useful for planning appoitnments)

What else should I add here ?
Thanks

If you want to see piles of “real situation” type maths, South Africa has the school subject “Maths Literacy” for the 90% of the population who can’t calculate anything even if their lives depended on it. It is actually a useful subject and there are many examination papers available online. Search for eg: “Gauteng maths literacy exam papers” ( Gauteng is a province ). Some examples ( look at the “_eng” ones for English, unless you read Afrikaans or Zulu or whatever ):

http://olivershouse.co.za/community-projects/educational-learning-centre/grade-12-past-exam-papers/
http://www.thutong.doe.gov.za/Default.aspx?alias=www.thutong.doe.gov.za/mathematicaliteracy

thanks for yr contributions but these are problems it’s a bit different from my question, I need simple questions that can be put into a multi choice test

Areas, especially of floors where doorways and alcoves will affect it.
Useful for ordering tiles and carpets.

Miles per gallon/ Km per litre for ‘how far will my car travel on this much fuel’

Speed and distance for ‘how long will a journey take’