What number makes this true 4 0.4 x?

Answer 1

Not written very well. I am assuming you have these 3 numbers and you want to keep it in the same pattern

4, 0.4, x

Mathematically I can tell you that you don't have enough information here.

I can immediately think of 2 answers (there may be more)

For example - using an arithmetic sequence to go from 4 to 0.4 you subtract 3.6 - now do it again and you get -3.2

Using a geometric sequence to go from 4 to 0.4 you divide by 10 - now do it again and you get 0.04

Of course you are not limited to those sequences but those are the most common.

So, honestly you don't have enough information but the 2 answers that stand out are -3.2 (if you were looking for an arithmetic sequence) and 0.04 if you were looking for a geometric sequence)