Each decimal place refers to that number divided by 10 to the power of the place.

So 0.1 = 10^-1 = 1 / 10^1 = 1 / 10

0.01 = 10^-2 = 1 / 10^2 = 1 / 100

0.001 = 10^-3 = 1 / 10^3 = 1 / 1000

etc…

So when you’re building your fraction, you can take this into account. Separate out the whole number part, then turn the decimal part into a fraction, then add them.

5.002 = 5 + 2/1000 = 5(1000/1000) + 2 / 1000 = 5000/1000 + 2/1000 = 5002/1000.

In your second example, 0.075 I think it was, you do the same sort of thing.

0.075 = 0 + 75 / 1000 = 75 / 1000

This you can simply a bit as 25 is a common factor in both the numerator and the demonimator.

This gives you 3/40.