How to multiply a number by 25 fast:

We know that we can write 25 as 100/4.

So the trick is to divide the number by 4 and place

1. '00' at ten's place and unit's place if the number is completely divisible by 4.
2. '25' at ten's place and unit's place if we get 1 as remainder.
3. '50' at ten's place and unit's place if we get 2 as remainder.
4. '75' at ten's place and unit's place if we get 3 as remainder.

Example 1:
Multiply 124 into 25

Step 1: Divide 124 by 4 = 31
Step 2: Place '00' in the end, we get 3100

Example 2:
Multiply 633 into 25

Step 1: Divide 633 by 4 = 158 with 1 as remainder.
Step 2: Place '25' in the end, we get 15825

We hope it helps. You can get more such simplification tricks on http://www.smartkeeda.com/Quantitative_Aptitude/Arithmetic/Simplification/newest/all/