Suppose all of the operations result in whole numbers. Explain why (a + b)+c = (a+c)+(b+c).
If both (a+c) and (b + c) result in whole numbers, that means (a+c) is a multiple of for some whole number x and (b+c) is a multiple of for some whole number y. By substitution and the distributive property of multiplication, this gives a + b = x + y. Using the definition of division, x + y = (a+c) + (b + c).
and x + y also equals (a+c) + (b + c)by substitution.