Problem Solving - 2

If the positive integers m and n have the same two digits, but in reverse order, then the difference between m and n cannot be:

a) 24

b) 36

c) 54

d) 63

e) 72

Answer - (a)

Let m consists of digits x & y,

thus m = 10x+y and

n = 10y+x

m-n = 9x-9y = m-n = 9(x-y)

=> m-n has to be a multiple of 9....