Mental Multiplication of Integers ending with 5

Multiplication of numbers ending in 5 can be easily done mentally. The processes are the following:

-for the product, one should remember that it will only end with either 25 or 75.
-How will I know that it will be 25 or 75? Let's say I will multiply 25 with 45.
-What I will do first is to multiply the numbers in the factors which are prior to 5, in this case, 2x4=8.
-Then, I will get the sum of the numbers I multiplied earlier and divide their sum by 2, in this case (2+4)/2=3. If the there is no decimal to this answer, the last two digits that I will choose is 25.
- Then I will add the first answer with the last answer, in this case, 8+3 =11.
- Since 25 are the last two digits, the product will be 1125.

-How about the product that will end with 75. Let's say I will multiply 25 with 35.
-I will do 2x3=6.
-Then (2+3)/2 = 2.5. Since there is decimal in this answer, the last two digits will be 75.
-Then I will do 6+2=8.
-Since 75 are the last two digits, the product will be 875.

More examples:

95x85=(9x8)+[9+8)/2]=80.5. Since with decimal, the final product is 8075.
135x75=(13x7)+[13+7)/2]=101. Since there is no decimal, the final product is 10,125.

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