There's a fancy expression called the "commutative property" that basically says this ( 45*23 = 23*45 ) is just the way math works. Proving it requires math skill beyond what you would get in fourth grade.
Think of it this way: How do we know 1 * 5 = 5 * 1 ? If you had one $5 bill, you have the same amount of money as if you had five $1 bills.