Let the 2 digit number be 10a + b, where a and b are the tens digit and the ones digit.
Given that
a + b = 13 (equation 1) and
10a + b —9 = 10b + a (equation 2)
So 9a = 9b + 9 , which means
a = b + 1 (equation 3)
Substitute (3) into (1)
(b + 1) + b = 13
2b = 12
b = 6 (equation 4)
Substitute (4) into (3)
a = 6 + 1 = 7 (equation 5)
The 2 digit number is 76.
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