If a number is broken in two parts then product of parts will increase as difference between them decreases.
Alternatively ,
Suppose we have a number c, and c is written as sum of a and b i.e., c=a+b
then a*b will increase as difference ( a- b) between a and b decrease.
for example take 9
9= 1+8, 8-1 = 7, 1*8 =8
9= 2+7, 7-2 = 5 , 2*7 =14
9 = 3+6, 6-3 = 3, 3*6 =18
9 =4+5, 5-4 =1 (min) , 4*5 =20 (max)
have a look at numbers
100210+ 90021= 100021 + 90210
and compare the differences
100210 - 90021 is greater than 100021 -90210
Hope it would work for you.