Ok, to start, an integer modulo x is the remainder when the integer is divided by x.
So, for example, 17 modulo 5 is 17/5's remainder, or 2.
I will now explain how to prove the statement.
An integer ending in 0 or 5 modulo 5 is 0
Ending in 1 or 6 modulo 5 is 1
Ending in 2 or 7 modulo 5 is 2
Ending in 3 or 8 modulo 5 is 3
Ending in 4 or 9 modulo 5 is 4
Now, I now must prove that no integer, when squared, ends in 2, 3, 7, or 8.
An integer ending in 0 sqaured ends in 0x0, or 0. Thus, it is 0 modulo 5.
Ending in 1: 1x1 = 1, so 1 modulo 5.
Ending in 2: 2x2 = 4, so 4 modulo 5.
Ending in 3: 3x3 = 9, so 4 modulo 5.
Ending in 4: 4x4 = 16, so it ends in 6 and is 1 modulo 5.
Ending in 5: 5x5 = 25, so it ends in 5 and is 0 modulo 5.
Ending in 6: 6x6 = 36, so it ends in 6 and is 1 modulo 5.
Ending in 7: 7x7 = 49, so it ends in 9 and is 4 modulo 5.
Ending in 8: 8x8 = 64, so it ends in 4 and is 4 modulo 5.
Ending in 9: 9x9 = 81, so it ends in 1 and is 1 modulo 5.
And the square of ANY integer's last digit is the last digit of the square of the integer.
Notice that none of the digits squared ends in 2, 3, 7, or 8. Thus:
The square of any integer modulo 5 CANNOT be 2 or 3.
I found that when I was bored one day.
Congratulations! You have just been blessed by the wisdom of Steve.