Mathematical trick

Trick 1 : Subtract in Bits
There's no need to subtract things all at once in your head. If you have to subtract 150 from something, you have 100 and 50 to subtract separately.
1100 - 150 → First of all, subtract the 100 to make it easier:
1100 - 150 = 1000
now all you have to do is subtract the remaining 50:
1000 - 50 = 950

Another example:

1450 -  125
I can either subtract the 100, the 20 and the 5 separately, but looking at it, I have decided to subtract the 100 and the 25 instead:
1450 - 100 = 1350
13 50 - 25 = 13 25 (I split them up in to pairs so I can think about them more easily, as explained above.
Just to explain last bit, I thought in my head:

 13 50
  -   25   
      25

This method is like the 'sliding rule' above, only thought about in a different way.