What is the easy way to multiply two numbers ending with 5?

Take75*25, for example. Add the numbers and divide by2. We get50, square it, we get2500.

Take the difference of two numbers and divide by2. We get25, square it, we get625.

2500-625=1875 is the answer.

(a+b)*(a-b)=a^2-b^2

In the above example a=50, b=25

so,75*25=50^2-25^2