Because multiplication of real numbers is Abelian (commutative). That is, for any two numbers, x and y, x*y = y*x
The multiples of 21 up to 500 are as follows: 21x1=21 21x2=42 21x3=63 21x4=84 21x5=105 21x6=126 21x7=147 21x8=168 21x9=189 21x10=210 21x11=231 21x12=252 21x13=273 21x14=294 21x15=315 21x16=336 21x17=357 21x18=378 21x19=399 21x20=420 21x21=441 21x22=462 21x23=483 21x24=504