Suppose subtract 358 from 10000.
Here Base 1000=10'3, 10 raised to power 3
How to calculate faster in mind, just subtract 3 from 9, 5 from 9 and last digit 8 from 10. Now we get 642. Isn't it so faster than regular maths. Just looking the number and you get the answer in mind, no pen and paper no calculation. Keep trying with other numbers.