How do you write 12.4 trillion in numbers?

Answer 1

That depends on what scale your country is using (long of short scale)

For the long scale 12.4 trillion would be 12'400'000'000'000'000'000

And for the short scale it would be 12'400'000'000'000

Information about short and long scales can be found on Wikipedia over here:

http://en.wikipedia.org/wiki/Long_and_short_scales