Question 875640
let n be the mystery number
a is the 10's digit
b is the one's digit
10a+b=n,
a+b=13,
2(n-2)= 10b+a
a = 4,   b = 9,   n = 49

we know that is is a number under 100
the digits add up to 13
4 9
5 8
6 7
7 6 
8 5 
9 4