Medium
The sum of the digits of a two-digit number added to the product of the digits, give the original number again. What is the digit in the one's place of the number?
Explanation
Explanation
Let the number be 10a+b (a=tens digit, b=units digit). The condition gives a+b+ab=10a+b, simplifying to ab=9a, so b=9 for any nonzero tens digit a. Hence the digit in the one's place is 9.
PRO TIP
Re-check your working by substituting the answer back into the original question.