(-0.4) + (-7.9)

$$(-0.4) + (-7.9) = -0.4\; +\; - \;7.9$$

I think of negative numbers as cold numbers and positive numbers as hot numbers.

Here you are adding cold to a room.   Does it get hotter or does it get colder?  It gets colder ofcourse!

So 

$$-0.4\; +\; - \;7.9 = -0.4-7.9$$

Now to do this you start at -0.4 in the number line and then move 7.9 units in the negative direction.

 

This is a bit trickier than subtraction with whole numbers  but if you think about this picture you may realise that  

 

$$-0.4-7.9=-(0.4+7.9)=-(8.3)=-8.3$$

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