MATH
posted by ANGIE .
ROBERT NUMBER IS BETWEEN 20 AND 32. THE SUM DIGITS IS 10. WHAT IS ROBERT NUMBER? I KNOW ANSWER IS 28 BUT NEED TO KNOW STEP BY STEP HOW TO GET TO 28?

Only one of these numbers has digits that add up to 10.
21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31