t = tens digit of N u = units digit of N 10t + u = N 10u + t = K N - K > 18 (10t + u) - (10u + t) > 18 10t + u - 10u - t > 18 9t - 9u > 18 t - u > 2 t > u + 2 Since t < 10, it must be that u < 7 Since N is odd, the units digit is u = 1,3, or 5 If u = 1, t > 3, so N = 41,51,61,71,81,91 If u = 3, t > 5, so N = 63,73,83,93 If u = 5, t > 7, so N = 85,95 Edwin