Question

A numbrr consists of two digits whose sum is 9. If 45 is subtracted from the number its digits are interchanged. Find the number

Answer 1

x+y=9
10x+y-45=10y+x , i.e. 9x-9y= 45 or x-y= 5

solving these equations we get
2x= 14, or x= 7 and so y=2

So the number is 72.

Answer 2

x+y=9. 10y+x=45
y=9-x.
10(9-x)+x=45
90-10x=45
9(10-1x)=45
10-x=45/9
x=10-5
x=5