x + y = 9 equation 1 the sum of the digits of a two-digit number is 9
the number formed by interchanging the digits is 27 less than the original number
we are looking at the number xy,
10x+y for x is a tens digit.
10y + x = 10x + y + 27
simplify it,
9y = 9x + 27 divide by 9
x + y = 9 equation 1
y= x + 3 equation 2
by subtitution we put y in equation 1
x + (x+3) = 9
2x = 6
x= 6/2= 3
y = x+3=3+3=6
