A number that ends in 5 can be written as 10x + 5 or 10y + 5. So we multiply those factors and re-group. The term xy + (x + y)/2 means to add the product and the average of the leading digits. Then you multiply by 100 and finally add 25.

1– Add the and average of leading digits

2- Multiply by 100

3-Add 25

Example 1

Example 2

Example 3

Example 4