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"If you add a multiple of 10 to a multiple of 5, the answer is always a multiple of 5."
Always true because 10 is a multiple of 5, therefore say if you had 25, if you add 20 onto it, which is a multiple of 10, you would get a multiple of 5 because it would be like mutliplying 5 x 4.
Because all mutliples of 10 have no ones in the ones column, when you add a multiple of 10 to a multiple of 5, you don't change the ones column so the ones column will always be 5 or 0 and all numbers that end in 5 or 0 are multiples of 5.