Multiplying together 2-digit numbers having 5 as the tens digit

To multiply together 2-digit numbers having 5 as the tens digit:    ex.  53×51=2703

-Multiply together the ones digits of the numbers being multiplied. This yields the last 2 digits of the answer.    ex. 3×1=_ _ 03

-Add the ones digits of the numbers being multiplied and divide by two. If this “average” digit is a whole number add it to the square of the common tens digit (5) of the numbers being multiplied. This yields the first 2 digits of the answer.    ex. (3+1)/2=2    5×5+2=27_ _

*If the “average” digit above is not even, it is not even by half a hundred. Use this       “average” figure disregarding the fraction, but adding 50 to the answer.    ex.  52×57=2964, where (2+7)/2=4 and fraction. Adding this 4 to the square of the common tens digit (5) yields 29 for the first 2 numbers of the answer. Because the “average” figure is off by half a hundred, add 50 to the last 2 digits of the answer.

*When doing this type of problem, know that if both ones digits in the problem are even ,  or if both are odd, the “average’ will be a whole number and no problem exists.  If the ones digits in the problem are a combination of even and odd, only then is 50 is added to the answer.

Consider:    54×54=2916      58×52=3016      51×58=2958      53×54=2862      52×59=3068

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