What should be subtracted from `15, 28, 20 and 38` so that the examining numbers maybe proportional?

What should be subtracted from .1 to get .04?

What should be subtracted from . 1 to get .03?

What should be subtracted from -2 to get -17 ?

Which number should be subtracted from 7,12 and 18 such that the resultant numbers are in continued proportion?

Which number should be subtracted from 12,16 and 21 so that resultant numbers are in continued proportion?

Which number should be subtracted from 13,25 and 55 so that resultant numbers are in continued proportion?