The sum of two digits number is 7. In changing position of digits we get the number 9 less than the original number. What is number?
If the digits are t and u, then the value of the number is 10t+u. So, we have
t+u=7
10u+t = 10t+u-9
Now just solve for t and u.
You can do this without any algebra. I mean, how many pairs of numbers add up to 7? Just try them.