When square of a number which ends with 5 is to be derived, we can find the square by simply multiplying the digits in other places excluding the unit's place by a number one more than the digits and substituting it preceding 25.

For instance. 35²=____25 to get the number which can fill the blank we will have to multiply 5 with the number 1 less than 3 i.e 3+1=4. (3x4=12). So  the square of 35 is 1225.