The only numbers that can be squared to end in 6 are numbers that end in 4 or 6.
We need to find the possible values of the tens digit, so lets let the tens digit be a variable such as x.
The last two digits of our number will look like this:
10x + 6
or this:
10x + 4
I find it easiest to just plug in values from (1-9) for x and see what you get.
If x is 1, the last two digits will be ...56 or ...96
If x is 2, the last two digits will be ...76 or ...76
If x is 3, the last two digits will be ...96 or ...56
If x is 4, the last two digits will be ...16 or ...36
If x is 5, the last two digits will be ...36 or ...16
.
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We can keep going all the way to x=9, and you will see that the tens digit will always be an odd number...
Therefore the possible values of the tens digit is (1, 3, 5 , 7, 9)