Which number is written in scientific notation? 7.8 x 10−5 0.84 x 106 3.4 x 1002 −5 x 10−12

Hello there, 

First off what is scientific notation?

Scientific Notation is the way that scientist handle very large or very small numbers..
for example, instead of writing 0.0000000056 we write 5.6x10 (exponent -9)

So, now we got that out the way lets look up here at the choices

I would say that 7.8x10 (exponent -5) would be the answer! :)

I hoped this helped :)))

God bless you.

7.8x10-5 is the answer

X> -2 is the simplified version.