What is the difference of #9.4-7.9#?

#9.4 - 7.9# = 1.5

You can check this answer by adding 7.9 to 1.5 and you will end up with 9.4.

#color(blue)("This is how the process should look and it is very quick:")#

#color(white)(..)cancel(9)^8 .cancel(4)^14#
#color(white)(..)ul(7color(white)(..).9)#
#color(white)(..)1color(white)(..).5#

#color(white)(..)#
#color(white)(..)#

'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#color(blue)("This is the very detailed explanation")#

Write as:
#" "9.4#
#" "ul(7.9) larr" Subtract"#

Work from right to left
'~~~~~~~~~~~~~~~~~~~~~~~~~~~
#color(green)("Subtracting the right column")#

Notice that the right hand column the 4 is less than the 9.

For subtracting this part we have #color(red)(9+4/10)color(blue)( -9/10)#

This is the same as #color(red)(8+14/10)color(blue)(-9/10)#

So in our subtraction of the right column, instead of having:

#color(white)(.)9color(magenta)(.4)#
#ul(color(white)(..) color(magenta)(.9)) larr" Subtract"#

We have:
#color(white)(.)8color(magenta)( .cancel(4)^14)#
#ul(color(white)(....) color(magenta)(.9)) larr" Subtract"#
#color(white)(....)color(magenta)(.5)#
'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#color(green)("Subtracting the left column")#

#color(white)(..)color(magenta)(8) .cancel(4)^14#
#ul(color(white)(..)color(magenta)(7) .9) larr" Subtract"#
#color(white)(...)color(magenta)(1).5#

#color(white)(..)#

9.4 - 7.9 = 1.5