If the sum of digits is 9 and the difference between the digit in the ten's place and unit's place is 1, then the two-digit number is?
Let the number at ten's place be x and unit's place be y
x + y = 9 -------I
x - y = 1 -------II
Adding I and II we get,
2x = 10
x = 5 and y = 4
Required number = 54.
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